Annotation of embedaddon/pcre/doc/pcre.txt, revision 1.1
1.1 ! misho 1: -----------------------------------------------------------------------------
! 2: This file contains a concatenation of the PCRE man pages, converted to plain
! 3: text format for ease of searching with a text editor, or for use on systems
! 4: that do not have a man page processor. The small individual files that give
! 5: synopses of each function in the library have not been included. Neither has
! 6: the pcredemo program. There are separate text files for the pcregrep and
! 7: pcretest commands.
! 8: -----------------------------------------------------------------------------
! 9:
! 10:
! 11: PCRE(3) PCRE(3)
! 12:
! 13:
! 14: NAME
! 15: PCRE - Perl-compatible regular expressions
! 16:
! 17:
! 18: INTRODUCTION
! 19:
! 20: The PCRE library is a set of functions that implement regular expres-
! 21: sion pattern matching using the same syntax and semantics as Perl, with
! 22: just a few differences. Some features that appeared in Python and PCRE
! 23: before they appeared in Perl are also available using the Python syn-
! 24: tax, there is some support for one or two .NET and Oniguruma syntax
! 25: items, and there is an option for requesting some minor changes that
! 26: give better JavaScript compatibility.
! 27:
! 28: The current implementation of PCRE corresponds approximately with Perl
! 29: 5.12, including support for UTF-8 encoded strings and Unicode general
! 30: category properties. However, UTF-8 and Unicode support has to be
! 31: explicitly enabled; it is not the default. The Unicode tables corre-
! 32: spond to Unicode release 6.0.0.
! 33:
! 34: In addition to the Perl-compatible matching function, PCRE contains an
! 35: alternative function that matches the same compiled patterns in a dif-
! 36: ferent way. In certain circumstances, the alternative function has some
! 37: advantages. For a discussion of the two matching algorithms, see the
! 38: pcrematching page.
! 39:
! 40: PCRE is written in C and released as a C library. A number of people
! 41: have written wrappers and interfaces of various kinds. In particular,
! 42: Google Inc. have provided a comprehensive C++ wrapper. This is now
! 43: included as part of the PCRE distribution. The pcrecpp page has details
! 44: of this interface. Other people's contributions can be found in the
! 45: Contrib directory at the primary FTP site, which is:
! 46:
! 47: ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
! 48:
! 49: Details of exactly which Perl regular expression features are and are
! 50: not supported by PCRE are given in separate documents. See the pcrepat-
! 51: tern and pcrecompat pages. There is a syntax summary in the pcresyntax
! 52: page.
! 53:
! 54: Some features of PCRE can be included, excluded, or changed when the
! 55: library is built. The pcre_config() function makes it possible for a
! 56: client to discover which features are available. The features them-
! 57: selves are described in the pcrebuild page. Documentation about build-
! 58: ing PCRE for various operating systems can be found in the README and
! 59: NON-UNIX-USE files in the source distribution.
! 60:
! 61: The library contains a number of undocumented internal functions and
! 62: data tables that are used by more than one of the exported external
! 63: functions, but which are not intended for use by external callers.
! 64: Their names all begin with "_pcre_", which hopefully will not provoke
! 65: any name clashes. In some environments, it is possible to control which
! 66: external symbols are exported when a shared library is built, and in
! 67: these cases the undocumented symbols are not exported.
! 68:
! 69:
! 70: USER DOCUMENTATION
! 71:
! 72: The user documentation for PCRE comprises a number of different sec-
! 73: tions. In the "man" format, each of these is a separate "man page". In
! 74: the HTML format, each is a separate page, linked from the index page.
! 75: In the plain text format, all the sections, except the pcredemo sec-
! 76: tion, are concatenated, for ease of searching. The sections are as fol-
! 77: lows:
! 78:
! 79: pcre this document
! 80: pcre-config show PCRE installation configuration information
! 81: pcreapi details of PCRE's native C API
! 82: pcrebuild options for building PCRE
! 83: pcrecallout details of the callout feature
! 84: pcrecompat discussion of Perl compatibility
! 85: pcrecpp details of the C++ wrapper
! 86: pcredemo a demonstration C program that uses PCRE
! 87: pcregrep description of the pcregrep command
! 88: pcrejit discussion of the just-in-time optimization support
! 89: pcrelimits details of size and other limits
! 90: pcrematching discussion of the two matching algorithms
! 91: pcrepartial details of the partial matching facility
! 92: pcrepattern syntax and semantics of supported
! 93: regular expressions
! 94: pcreperform discussion of performance issues
! 95: pcreposix the POSIX-compatible C API
! 96: pcreprecompile details of saving and re-using precompiled patterns
! 97: pcresample discussion of the pcredemo program
! 98: pcrestack discussion of stack usage
! 99: pcresyntax quick syntax reference
! 100: pcretest description of the pcretest testing command
! 101: pcreunicode discussion of Unicode and UTF-8 support
! 102:
! 103: In addition, in the "man" and HTML formats, there is a short page for
! 104: each C library function, listing its arguments and results.
! 105:
! 106:
! 107: AUTHOR
! 108:
! 109: Philip Hazel
! 110: University Computing Service
! 111: Cambridge CB2 3QH, England.
! 112:
! 113: Putting an actual email address here seems to have been a spam magnet,
! 114: so I've taken it away. If you want to email me, use my two initials,
! 115: followed by the two digits 10, at the domain cam.ac.uk.
! 116:
! 117:
! 118: REVISION
! 119:
! 120: Last updated: 24 August 2011
! 121: Copyright (c) 1997-2011 University of Cambridge.
! 122: ------------------------------------------------------------------------------
! 123:
! 124:
! 125: PCREBUILD(3) PCREBUILD(3)
! 126:
! 127:
! 128: NAME
! 129: PCRE - Perl-compatible regular expressions
! 130:
! 131:
! 132: PCRE BUILD-TIME OPTIONS
! 133:
! 134: This document describes the optional features of PCRE that can be
! 135: selected when the library is compiled. It assumes use of the configure
! 136: script, where the optional features are selected or deselected by pro-
! 137: viding options to configure before running the make command. However,
! 138: the same options can be selected in both Unix-like and non-Unix-like
! 139: environments using the GUI facility of cmake-gui if you are using CMake
! 140: instead of configure to build PCRE.
! 141:
! 142: There is a lot more information about building PCRE in non-Unix-like
! 143: environments in the file called NON_UNIX_USE, which is part of the PCRE
! 144: distribution. You should consult this file as well as the README file
! 145: if you are building in a non-Unix-like environment.
! 146:
! 147: The complete list of options for configure (which includes the standard
! 148: ones such as the selection of the installation directory) can be
! 149: obtained by running
! 150:
! 151: ./configure --help
! 152:
! 153: The following sections include descriptions of options whose names
! 154: begin with --enable or --disable. These settings specify changes to the
! 155: defaults for the configure command. Because of the way that configure
! 156: works, --enable and --disable always come in pairs, so the complemen-
! 157: tary option always exists as well, but as it specifies the default, it
! 158: is not described.
! 159:
! 160:
! 161: BUILDING SHARED AND STATIC LIBRARIES
! 162:
! 163: The PCRE building process uses libtool to build both shared and static
! 164: Unix libraries by default. You can suppress one of these by adding one
! 165: of
! 166:
! 167: --disable-shared
! 168: --disable-static
! 169:
! 170: to the configure command, as required.
! 171:
! 172:
! 173: C++ SUPPORT
! 174:
! 175: By default, the configure script will search for a C++ compiler and C++
! 176: header files. If it finds them, it automatically builds the C++ wrapper
! 177: library for PCRE. You can disable this by adding
! 178:
! 179: --disable-cpp
! 180:
! 181: to the configure command.
! 182:
! 183:
! 184: UTF-8 SUPPORT
! 185:
! 186: To build PCRE with support for UTF-8 Unicode character strings, add
! 187:
! 188: --enable-utf8
! 189:
! 190: to the configure command. Of itself, this does not make PCRE treat
! 191: strings as UTF-8. As well as compiling PCRE with this option, you also
! 192: have have to set the PCRE_UTF8 option when you call the pcre_compile()
! 193: or pcre_compile2() functions.
! 194:
! 195: If you set --enable-utf8 when compiling in an EBCDIC environment, PCRE
! 196: expects its input to be either ASCII or UTF-8 (depending on the runtime
! 197: option). It is not possible to support both EBCDIC and UTF-8 codes in
! 198: the same version of the library. Consequently, --enable-utf8 and
! 199: --enable-ebcdic are mutually exclusive.
! 200:
! 201:
! 202: UNICODE CHARACTER PROPERTY SUPPORT
! 203:
! 204: UTF-8 support allows PCRE to process character values greater than 255
! 205: in the strings that it handles. On its own, however, it does not pro-
! 206: vide any facilities for accessing the properties of such characters. If
! 207: you want to be able to use the pattern escapes \P, \p, and \X, which
! 208: refer to Unicode character properties, you must add
! 209:
! 210: --enable-unicode-properties
! 211:
! 212: to the configure command. This implies UTF-8 support, even if you have
! 213: not explicitly requested it.
! 214:
! 215: Including Unicode property support adds around 30K of tables to the
! 216: PCRE library. Only the general category properties such as Lu and Nd
! 217: are supported. Details are given in the pcrepattern documentation.
! 218:
! 219:
! 220: JUST-IN-TIME COMPILER SUPPORT
! 221:
! 222: Just-in-time compiler support is included in the build by specifying
! 223:
! 224: --enable-jit
! 225:
! 226: This support is available only for certain hardware architectures. If
! 227: this option is set for an unsupported architecture, a compile time
! 228: error occurs. See the pcrejit documentation for a discussion of JIT
! 229: usage. When JIT support is enabled, pcregrep automatically makes use of
! 230: it, unless you add
! 231:
! 232: --disable-pcregrep-jit
! 233:
! 234: to the "configure" command.
! 235:
! 236:
! 237: CODE VALUE OF NEWLINE
! 238:
! 239: By default, PCRE interprets the linefeed (LF) character as indicating
! 240: the end of a line. This is the normal newline character on Unix-like
! 241: systems. You can compile PCRE to use carriage return (CR) instead, by
! 242: adding
! 243:
! 244: --enable-newline-is-cr
! 245:
! 246: to the configure command. There is also a --enable-newline-is-lf
! 247: option, which explicitly specifies linefeed as the newline character.
! 248:
! 249: Alternatively, you can specify that line endings are to be indicated by
! 250: the two character sequence CRLF. If you want this, add
! 251:
! 252: --enable-newline-is-crlf
! 253:
! 254: to the configure command. There is a fourth option, specified by
! 255:
! 256: --enable-newline-is-anycrlf
! 257:
! 258: which causes PCRE to recognize any of the three sequences CR, LF, or
! 259: CRLF as indicating a line ending. Finally, a fifth option, specified by
! 260:
! 261: --enable-newline-is-any
! 262:
! 263: causes PCRE to recognize any Unicode newline sequence.
! 264:
! 265: Whatever line ending convention is selected when PCRE is built can be
! 266: overridden when the library functions are called. At build time it is
! 267: conventional to use the standard for your operating system.
! 268:
! 269:
! 270: WHAT \R MATCHES
! 271:
! 272: By default, the sequence \R in a pattern matches any Unicode newline
! 273: sequence, whatever has been selected as the line ending sequence. If
! 274: you specify
! 275:
! 276: --enable-bsr-anycrlf
! 277:
! 278: the default is changed so that \R matches only CR, LF, or CRLF. What-
! 279: ever is selected when PCRE is built can be overridden when the library
! 280: functions are called.
! 281:
! 282:
! 283: POSIX MALLOC USAGE
! 284:
! 285: When PCRE is called through the POSIX interface (see the pcreposix doc-
! 286: umentation), additional working storage is required for holding the
! 287: pointers to capturing substrings, because PCRE requires three integers
! 288: per substring, whereas the POSIX interface provides only two. If the
! 289: number of expected substrings is small, the wrapper function uses space
! 290: on the stack, because this is faster than using malloc() for each call.
! 291: The default threshold above which the stack is no longer used is 10; it
! 292: can be changed by adding a setting such as
! 293:
! 294: --with-posix-malloc-threshold=20
! 295:
! 296: to the configure command.
! 297:
! 298:
! 299: HANDLING VERY LARGE PATTERNS
! 300:
! 301: Within a compiled pattern, offset values are used to point from one
! 302: part to another (for example, from an opening parenthesis to an alter-
! 303: nation metacharacter). By default, two-byte values are used for these
! 304: offsets, leading to a maximum size for a compiled pattern of around
! 305: 64K. This is sufficient to handle all but the most gigantic patterns.
! 306: Nevertheless, some people do want to process truyl enormous patterns,
! 307: so it is possible to compile PCRE to use three-byte or four-byte off-
! 308: sets by adding a setting such as
! 309:
! 310: --with-link-size=3
! 311:
! 312: to the configure command. The value given must be 2, 3, or 4. Using
! 313: longer offsets slows down the operation of PCRE because it has to load
! 314: additional bytes when handling them.
! 315:
! 316:
! 317: AVOIDING EXCESSIVE STACK USAGE
! 318:
! 319: When matching with the pcre_exec() function, PCRE implements backtrack-
! 320: ing by making recursive calls to an internal function called match().
! 321: In environments where the size of the stack is limited, this can se-
! 322: verely limit PCRE's operation. (The Unix environment does not usually
! 323: suffer from this problem, but it may sometimes be necessary to increase
! 324: the maximum stack size. There is a discussion in the pcrestack docu-
! 325: mentation.) An alternative approach to recursion that uses memory from
! 326: the heap to remember data, instead of using recursive function calls,
! 327: has been implemented to work round the problem of limited stack size.
! 328: If you want to build a version of PCRE that works this way, add
! 329:
! 330: --disable-stack-for-recursion
! 331:
! 332: to the configure command. With this configuration, PCRE will use the
! 333: pcre_stack_malloc and pcre_stack_free variables to call memory manage-
! 334: ment functions. By default these point to malloc() and free(), but you
! 335: can replace the pointers so that your own functions are used instead.
! 336:
! 337: Separate functions are provided rather than using pcre_malloc and
! 338: pcre_free because the usage is very predictable: the block sizes
! 339: requested are always the same, and the blocks are always freed in
! 340: reverse order. A calling program might be able to implement optimized
! 341: functions that perform better than malloc() and free(). PCRE runs
! 342: noticeably more slowly when built in this way. This option affects only
! 343: the pcre_exec() function; it is not relevant for pcre_dfa_exec().
! 344:
! 345:
! 346: LIMITING PCRE RESOURCE USAGE
! 347:
! 348: Internally, PCRE has a function called match(), which it calls repeat-
! 349: edly (sometimes recursively) when matching a pattern with the
! 350: pcre_exec() function. By controlling the maximum number of times this
! 351: function may be called during a single matching operation, a limit can
! 352: be placed on the resources used by a single call to pcre_exec(). The
! 353: limit can be changed at run time, as described in the pcreapi documen-
! 354: tation. The default is 10 million, but this can be changed by adding a
! 355: setting such as
! 356:
! 357: --with-match-limit=500000
! 358:
! 359: to the configure command. This setting has no effect on the
! 360: pcre_dfa_exec() matching function.
! 361:
! 362: In some environments it is desirable to limit the depth of recursive
! 363: calls of match() more strictly than the total number of calls, in order
! 364: to restrict the maximum amount of stack (or heap, if --disable-stack-
! 365: for-recursion is specified) that is used. A second limit controls this;
! 366: it defaults to the value that is set for --with-match-limit, which
! 367: imposes no additional constraints. However, you can set a lower limit
! 368: by adding, for example,
! 369:
! 370: --with-match-limit-recursion=10000
! 371:
! 372: to the configure command. This value can also be overridden at run
! 373: time.
! 374:
! 375:
! 376: CREATING CHARACTER TABLES AT BUILD TIME
! 377:
! 378: PCRE uses fixed tables for processing characters whose code values are
! 379: less than 256. By default, PCRE is built with a set of tables that are
! 380: distributed in the file pcre_chartables.c.dist. These tables are for
! 381: ASCII codes only. If you add
! 382:
! 383: --enable-rebuild-chartables
! 384:
! 385: to the configure command, the distributed tables are no longer used.
! 386: Instead, a program called dftables is compiled and run. This outputs
! 387: the source for new set of tables, created in the default locale of your
! 388: C runtime system. (This method of replacing the tables does not work if
! 389: you are cross compiling, because dftables is run on the local host. If
! 390: you need to create alternative tables when cross compiling, you will
! 391: have to do so "by hand".)
! 392:
! 393:
! 394: USING EBCDIC CODE
! 395:
! 396: PCRE assumes by default that it will run in an environment where the
! 397: character code is ASCII (or Unicode, which is a superset of ASCII).
! 398: This is the case for most computer operating systems. PCRE can, how-
! 399: ever, be compiled to run in an EBCDIC environment by adding
! 400:
! 401: --enable-ebcdic
! 402:
! 403: to the configure command. This setting implies --enable-rebuild-charta-
! 404: bles. You should only use it if you know that you are in an EBCDIC
! 405: environment (for example, an IBM mainframe operating system). The
! 406: --enable-ebcdic option is incompatible with --enable-utf8.
! 407:
! 408:
! 409: PCREGREP OPTIONS FOR COMPRESSED FILE SUPPORT
! 410:
! 411: By default, pcregrep reads all files as plain text. You can build it so
! 412: that it recognizes files whose names end in .gz or .bz2, and reads them
! 413: with libz or libbz2, respectively, by adding one or both of
! 414:
! 415: --enable-pcregrep-libz
! 416: --enable-pcregrep-libbz2
! 417:
! 418: to the configure command. These options naturally require that the rel-
! 419: evant libraries are installed on your system. Configuration will fail
! 420: if they are not.
! 421:
! 422:
! 423: PCREGREP BUFFER SIZE
! 424:
! 425: pcregrep uses an internal buffer to hold a "window" on the file it is
! 426: scanning, in order to be able to output "before" and "after" lines when
! 427: it finds a match. The size of the buffer is controlled by a parameter
! 428: whose default value is 20K. The buffer itself is three times this size,
! 429: but because of the way it is used for holding "before" lines, the long-
! 430: est line that is guaranteed to be processable is the parameter size.
! 431: You can change the default parameter value by adding, for example,
! 432:
! 433: --with-pcregrep-bufsize=50K
! 434:
! 435: to the configure command. The caller of pcregrep can, however, override
! 436: this value by specifying a run-time option.
! 437:
! 438:
! 439: PCRETEST OPTION FOR LIBREADLINE SUPPORT
! 440:
! 441: If you add
! 442:
! 443: --enable-pcretest-libreadline
! 444:
! 445: to the configure command, pcretest is linked with the libreadline
! 446: library, and when its input is from a terminal, it reads it using the
! 447: readline() function. This provides line-editing and history facilities.
! 448: Note that libreadline is GPL-licensed, so if you distribute a binary of
! 449: pcretest linked in this way, there may be licensing issues.
! 450:
! 451: Setting this option causes the -lreadline option to be added to the
! 452: pcretest build. In many operating environments with a sytem-installed
! 453: libreadline this is sufficient. However, in some environments (e.g. if
! 454: an unmodified distribution version of readline is in use), some extra
! 455: configuration may be necessary. The INSTALL file for libreadline says
! 456: this:
! 457:
! 458: "Readline uses the termcap functions, but does not link with the
! 459: termcap or curses library itself, allowing applications which link
! 460: with readline the to choose an appropriate library."
! 461:
! 462: If your environment has not been set up so that an appropriate library
! 463: is automatically included, you may need to add something like
! 464:
! 465: LIBS="-ncurses"
! 466:
! 467: immediately before the configure command.
! 468:
! 469:
! 470: SEE ALSO
! 471:
! 472: pcreapi(3), pcre_config(3).
! 473:
! 474:
! 475: AUTHOR
! 476:
! 477: Philip Hazel
! 478: University Computing Service
! 479: Cambridge CB2 3QH, England.
! 480:
! 481:
! 482: REVISION
! 483:
! 484: Last updated: 06 September 2011
! 485: Copyright (c) 1997-2011 University of Cambridge.
! 486: ------------------------------------------------------------------------------
! 487:
! 488:
! 489: PCREMATCHING(3) PCREMATCHING(3)
! 490:
! 491:
! 492: NAME
! 493: PCRE - Perl-compatible regular expressions
! 494:
! 495:
! 496: PCRE MATCHING ALGORITHMS
! 497:
! 498: This document describes the two different algorithms that are available
! 499: in PCRE for matching a compiled regular expression against a given sub-
! 500: ject string. The "standard" algorithm is the one provided by the
! 501: pcre_exec() function. This works in the same was as Perl's matching
! 502: function, and provides a Perl-compatible matching operation.
! 503:
! 504: An alternative algorithm is provided by the pcre_dfa_exec() function;
! 505: this operates in a different way, and is not Perl-compatible. It has
! 506: advantages and disadvantages compared with the standard algorithm, and
! 507: these are described below.
! 508:
! 509: When there is only one possible way in which a given subject string can
! 510: match a pattern, the two algorithms give the same answer. A difference
! 511: arises, however, when there are multiple possibilities. For example, if
! 512: the pattern
! 513:
! 514: ^<.*>
! 515:
! 516: is matched against the string
! 517:
! 518: <something> <something else> <something further>
! 519:
! 520: there are three possible answers. The standard algorithm finds only one
! 521: of them, whereas the alternative algorithm finds all three.
! 522:
! 523:
! 524: REGULAR EXPRESSIONS AS TREES
! 525:
! 526: The set of strings that are matched by a regular expression can be rep-
! 527: resented as a tree structure. An unlimited repetition in the pattern
! 528: makes the tree of infinite size, but it is still a tree. Matching the
! 529: pattern to a given subject string (from a given starting point) can be
! 530: thought of as a search of the tree. There are two ways to search a
! 531: tree: depth-first and breadth-first, and these correspond to the two
! 532: matching algorithms provided by PCRE.
! 533:
! 534:
! 535: THE STANDARD MATCHING ALGORITHM
! 536:
! 537: In the terminology of Jeffrey Friedl's book "Mastering Regular Expres-
! 538: sions", the standard algorithm is an "NFA algorithm". It conducts a
! 539: depth-first search of the pattern tree. That is, it proceeds along a
! 540: single path through the tree, checking that the subject matches what is
! 541: required. When there is a mismatch, the algorithm tries any alterna-
! 542: tives at the current point, and if they all fail, it backs up to the
! 543: previous branch point in the tree, and tries the next alternative
! 544: branch at that level. This often involves backing up (moving to the
! 545: left) in the subject string as well. The order in which repetition
! 546: branches are tried is controlled by the greedy or ungreedy nature of
! 547: the quantifier.
! 548:
! 549: If a leaf node is reached, a matching string has been found, and at
! 550: that point the algorithm stops. Thus, if there is more than one possi-
! 551: ble match, this algorithm returns the first one that it finds. Whether
! 552: this is the shortest, the longest, or some intermediate length depends
! 553: on the way the greedy and ungreedy repetition quantifiers are specified
! 554: in the pattern.
! 555:
! 556: Because it ends up with a single path through the tree, it is rela-
! 557: tively straightforward for this algorithm to keep track of the sub-
! 558: strings that are matched by portions of the pattern in parentheses.
! 559: This provides support for capturing parentheses and back references.
! 560:
! 561:
! 562: THE ALTERNATIVE MATCHING ALGORITHM
! 563:
! 564: This algorithm conducts a breadth-first search of the tree. Starting
! 565: from the first matching point in the subject, it scans the subject
! 566: string from left to right, once, character by character, and as it does
! 567: this, it remembers all the paths through the tree that represent valid
! 568: matches. In Friedl's terminology, this is a kind of "DFA algorithm",
! 569: though it is not implemented as a traditional finite state machine (it
! 570: keeps multiple states active simultaneously).
! 571:
! 572: Although the general principle of this matching algorithm is that it
! 573: scans the subject string only once, without backtracking, there is one
! 574: exception: when a lookaround assertion is encountered, the characters
! 575: following or preceding the current point have to be independently
! 576: inspected.
! 577:
! 578: The scan continues until either the end of the subject is reached, or
! 579: there are no more unterminated paths. At this point, terminated paths
! 580: represent the different matching possibilities (if there are none, the
! 581: match has failed). Thus, if there is more than one possible match,
! 582: this algorithm finds all of them, and in particular, it finds the long-
! 583: est. The matches are returned in decreasing order of length. There is
! 584: an option to stop the algorithm after the first match (which is neces-
! 585: sarily the shortest) is found.
! 586:
! 587: Note that all the matches that are found start at the same point in the
! 588: subject. If the pattern
! 589:
! 590: cat(er(pillar)?)?
! 591:
! 592: is matched against the string "the caterpillar catchment", the result
! 593: will be the three strings "caterpillar", "cater", and "cat" that start
! 594: at the fifth character of the subject. The algorithm does not automati-
! 595: cally move on to find matches that start at later positions.
! 596:
! 597: There are a number of features of PCRE regular expressions that are not
! 598: supported by the alternative matching algorithm. They are as follows:
! 599:
! 600: 1. Because the algorithm finds all possible matches, the greedy or
! 601: ungreedy nature of repetition quantifiers is not relevant. Greedy and
! 602: ungreedy quantifiers are treated in exactly the same way. However, pos-
! 603: sessive quantifiers can make a difference when what follows could also
! 604: match what is quantified, for example in a pattern like this:
! 605:
! 606: ^a++\w!
! 607:
! 608: This pattern matches "aaab!" but not "aaa!", which would be matched by
! 609: a non-possessive quantifier. Similarly, if an atomic group is present,
! 610: it is matched as if it were a standalone pattern at the current point,
! 611: and the longest match is then "locked in" for the rest of the overall
! 612: pattern.
! 613:
! 614: 2. When dealing with multiple paths through the tree simultaneously, it
! 615: is not straightforward to keep track of captured substrings for the
! 616: different matching possibilities, and PCRE's implementation of this
! 617: algorithm does not attempt to do this. This means that no captured sub-
! 618: strings are available.
! 619:
! 620: 3. Because no substrings are captured, back references within the pat-
! 621: tern are not supported, and cause errors if encountered.
! 622:
! 623: 4. For the same reason, conditional expressions that use a backrefer-
! 624: ence as the condition or test for a specific group recursion are not
! 625: supported.
! 626:
! 627: 5. Because many paths through the tree may be active, the \K escape
! 628: sequence, which resets the start of the match when encountered (but may
! 629: be on some paths and not on others), is not supported. It causes an
! 630: error if encountered.
! 631:
! 632: 6. Callouts are supported, but the value of the capture_top field is
! 633: always 1, and the value of the capture_last field is always -1.
! 634:
! 635: 7. The \C escape sequence, which (in the standard algorithm) matches a
! 636: single byte, even in UTF-8 mode, is not supported in UTF-8 mode,
! 637: because the alternative algorithm moves through the subject string one
! 638: character at a time, for all active paths through the tree.
! 639:
! 640: 8. Except for (*FAIL), the backtracking control verbs such as (*PRUNE)
! 641: are not supported. (*FAIL) is supported, and behaves like a failing
! 642: negative assertion.
! 643:
! 644:
! 645: ADVANTAGES OF THE ALTERNATIVE ALGORITHM
! 646:
! 647: Using the alternative matching algorithm provides the following advan-
! 648: tages:
! 649:
! 650: 1. All possible matches (at a single point in the subject) are automat-
! 651: ically found, and in particular, the longest match is found. To find
! 652: more than one match using the standard algorithm, you have to do kludgy
! 653: things with callouts.
! 654:
! 655: 2. Because the alternative algorithm scans the subject string just
! 656: once, and never needs to backtrack, it is possible to pass very long
! 657: subject strings to the matching function in several pieces, checking
! 658: for partial matching each time. Although it is possible to do multi-
! 659: segment matching using the standard algorithm (pcre_exec()), by retain-
! 660: ing partially matched substrings, it is more complicated. The pcrepar-
! 661: tial documentation gives details of partial matching and discusses
! 662: multi-segment matching.
! 663:
! 664:
! 665: DISADVANTAGES OF THE ALTERNATIVE ALGORITHM
! 666:
! 667: The alternative algorithm suffers from a number of disadvantages:
! 668:
! 669: 1. It is substantially slower than the standard algorithm. This is
! 670: partly because it has to search for all possible matches, but is also
! 671: because it is less susceptible to optimization.
! 672:
! 673: 2. Capturing parentheses and back references are not supported.
! 674:
! 675: 3. Although atomic groups are supported, their use does not provide the
! 676: performance advantage that it does for the standard algorithm.
! 677:
! 678:
! 679: AUTHOR
! 680:
! 681: Philip Hazel
! 682: University Computing Service
! 683: Cambridge CB2 3QH, England.
! 684:
! 685:
! 686: REVISION
! 687:
! 688: Last updated: 19 November 2011
! 689: Copyright (c) 1997-2010 University of Cambridge.
! 690: ------------------------------------------------------------------------------
! 691:
! 692:
! 693: PCREAPI(3) PCREAPI(3)
! 694:
! 695:
! 696: NAME
! 697: PCRE - Perl-compatible regular expressions
! 698:
! 699:
! 700: PCRE NATIVE API BASIC FUNCTIONS
! 701:
! 702: #include <pcre.h>
! 703:
! 704: pcre *pcre_compile(const char *pattern, int options,
! 705: const char **errptr, int *erroffset,
! 706: const unsigned char *tableptr);
! 707:
! 708: pcre *pcre_compile2(const char *pattern, int options,
! 709: int *errorcodeptr,
! 710: const char **errptr, int *erroffset,
! 711: const unsigned char *tableptr);
! 712:
! 713: pcre_extra *pcre_study(const pcre *code, int options,
! 714: const char **errptr);
! 715:
! 716: void pcre_free_study(pcre_extra *extra);
! 717:
! 718: int pcre_exec(const pcre *code, const pcre_extra *extra,
! 719: const char *subject, int length, int startoffset,
! 720: int options, int *ovector, int ovecsize);
! 721:
! 722:
! 723: PCRE NATIVE API AUXILIARY FUNCTIONS
! 724:
! 725: pcre_jit_stack *pcre_jit_stack_alloc(int startsize, int maxsize);
! 726:
! 727: void pcre_jit_stack_free(pcre_jit_stack *stack);
! 728:
! 729: void pcre_assign_jit_stack(pcre_extra *extra,
! 730: pcre_jit_callback callback, void *data);
! 731:
! 732: int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
! 733: const char *subject, int length, int startoffset,
! 734: int options, int *ovector, int ovecsize,
! 735: int *workspace, int wscount);
! 736:
! 737: int pcre_copy_named_substring(const pcre *code,
! 738: const char *subject, int *ovector,
! 739: int stringcount, const char *stringname,
! 740: char *buffer, int buffersize);
! 741:
! 742: int pcre_copy_substring(const char *subject, int *ovector,
! 743: int stringcount, int stringnumber, char *buffer,
! 744: int buffersize);
! 745:
! 746: int pcre_get_named_substring(const pcre *code,
! 747: const char *subject, int *ovector,
! 748: int stringcount, const char *stringname,
! 749: const char **stringptr);
! 750:
! 751: int pcre_get_stringnumber(const pcre *code,
! 752: const char *name);
! 753:
! 754: int pcre_get_stringtable_entries(const pcre *code,
! 755: const char *name, char **first, char **last);
! 756:
! 757: int pcre_get_substring(const char *subject, int *ovector,
! 758: int stringcount, int stringnumber,
! 759: const char **stringptr);
! 760:
! 761: int pcre_get_substring_list(const char *subject,
! 762: int *ovector, int stringcount, const char ***listptr);
! 763:
! 764: void pcre_free_substring(const char *stringptr);
! 765:
! 766: void pcre_free_substring_list(const char **stringptr);
! 767:
! 768: const unsigned char *pcre_maketables(void);
! 769:
! 770: int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
! 771: int what, void *where);
! 772:
! 773: int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
! 774:
! 775: int pcre_refcount(pcre *code, int adjust);
! 776:
! 777: int pcre_config(int what, void *where);
! 778:
! 779: char *pcre_version(void);
! 780:
! 781:
! 782: PCRE NATIVE API INDIRECTED FUNCTIONS
! 783:
! 784: void *(*pcre_malloc)(size_t);
! 785:
! 786: void (*pcre_free)(void *);
! 787:
! 788: void *(*pcre_stack_malloc)(size_t);
! 789:
! 790: void (*pcre_stack_free)(void *);
! 791:
! 792: int (*pcre_callout)(pcre_callout_block *);
! 793:
! 794:
! 795: PCRE API OVERVIEW
! 796:
! 797: PCRE has its own native API, which is described in this document. There
! 798: are also some wrapper functions that correspond to the POSIX regular
! 799: expression API, but they do not give access to all the functionality.
! 800: They are described in the pcreposix documentation. Both of these APIs
! 801: define a set of C function calls. A C++ wrapper is also distributed
! 802: with PCRE. It is documented in the pcrecpp page.
! 803:
! 804: The native API C function prototypes are defined in the header file
! 805: pcre.h, and on Unix systems the library itself is called libpcre. It
! 806: can normally be accessed by adding -lpcre to the command for linking an
! 807: application that uses PCRE. The header file defines the macros
! 808: PCRE_MAJOR and PCRE_MINOR to contain the major and minor release num-
! 809: bers for the library. Applications can use these to include support
! 810: for different releases of PCRE.
! 811:
! 812: In a Windows environment, if you want to statically link an application
! 813: program against a non-dll pcre.a file, you must define PCRE_STATIC
! 814: before including pcre.h or pcrecpp.h, because otherwise the pcre_mal-
! 815: loc() and pcre_free() exported functions will be declared
! 816: __declspec(dllimport), with unwanted results.
! 817:
! 818: The functions pcre_compile(), pcre_compile2(), pcre_study(), and
! 819: pcre_exec() are used for compiling and matching regular expressions in
! 820: a Perl-compatible manner. A sample program that demonstrates the sim-
! 821: plest way of using them is provided in the file called pcredemo.c in
! 822: the PCRE source distribution. A listing of this program is given in the
! 823: pcredemo documentation, and the pcresample documentation describes how
! 824: to compile and run it.
! 825:
! 826: Just-in-time compiler support is an optional feature of PCRE that can
! 827: be built in appropriate hardware environments. It greatly speeds up the
! 828: matching performance of many patterns. Simple programs can easily
! 829: request that it be used if available, by setting an option that is
! 830: ignored when it is not relevant. More complicated programs might need
! 831: to make use of the functions pcre_jit_stack_alloc(),
! 832: pcre_jit_stack_free(), and pcre_assign_jit_stack() in order to control
! 833: the JIT code's memory usage. These functions are discussed in the
! 834: pcrejit documentation.
! 835:
! 836: A second matching function, pcre_dfa_exec(), which is not Perl-compati-
! 837: ble, is also provided. This uses a different algorithm for the match-
! 838: ing. The alternative algorithm finds all possible matches (at a given
! 839: point in the subject), and scans the subject just once (unless there
! 840: are lookbehind assertions). However, this algorithm does not return
! 841: captured substrings. A description of the two matching algorithms and
! 842: their advantages and disadvantages is given in the pcrematching docu-
! 843: mentation.
! 844:
! 845: In addition to the main compiling and matching functions, there are
! 846: convenience functions for extracting captured substrings from a subject
! 847: string that is matched by pcre_exec(). They are:
! 848:
! 849: pcre_copy_substring()
! 850: pcre_copy_named_substring()
! 851: pcre_get_substring()
! 852: pcre_get_named_substring()
! 853: pcre_get_substring_list()
! 854: pcre_get_stringnumber()
! 855: pcre_get_stringtable_entries()
! 856:
! 857: pcre_free_substring() and pcre_free_substring_list() are also provided,
! 858: to free the memory used for extracted strings.
! 859:
! 860: The function pcre_maketables() is used to build a set of character
! 861: tables in the current locale for passing to pcre_compile(),
! 862: pcre_exec(), or pcre_dfa_exec(). This is an optional facility that is
! 863: provided for specialist use. Most commonly, no special tables are
! 864: passed, in which case internal tables that are generated when PCRE is
! 865: built are used.
! 866:
! 867: The function pcre_fullinfo() is used to find out information about a
! 868: compiled pattern; pcre_info() is an obsolete version that returns only
! 869: some of the available information, but is retained for backwards com-
! 870: patibility. The function pcre_version() returns a pointer to a string
! 871: containing the version of PCRE and its date of release.
! 872:
! 873: The function pcre_refcount() maintains a reference count in a data
! 874: block containing a compiled pattern. This is provided for the benefit
! 875: of object-oriented applications.
! 876:
! 877: The global variables pcre_malloc and pcre_free initially contain the
! 878: entry points of the standard malloc() and free() functions, respec-
! 879: tively. PCRE calls the memory management functions via these variables,
! 880: so a calling program can replace them if it wishes to intercept the
! 881: calls. This should be done before calling any PCRE functions.
! 882:
! 883: The global variables pcre_stack_malloc and pcre_stack_free are also
! 884: indirections to memory management functions. These special functions
! 885: are used only when PCRE is compiled to use the heap for remembering
! 886: data, instead of recursive function calls, when running the pcre_exec()
! 887: function. See the pcrebuild documentation for details of how to do
! 888: this. It is a non-standard way of building PCRE, for use in environ-
! 889: ments that have limited stacks. Because of the greater use of memory
! 890: management, it runs more slowly. Separate functions are provided so
! 891: that special-purpose external code can be used for this case. When
! 892: used, these functions are always called in a stack-like manner (last
! 893: obtained, first freed), and always for memory blocks of the same size.
! 894: There is a discussion about PCRE's stack usage in the pcrestack docu-
! 895: mentation.
! 896:
! 897: The global variable pcre_callout initially contains NULL. It can be set
! 898: by the caller to a "callout" function, which PCRE will then call at
! 899: specified points during a matching operation. Details are given in the
! 900: pcrecallout documentation.
! 901:
! 902:
! 903: NEWLINES
! 904:
! 905: PCRE supports five different conventions for indicating line breaks in
! 906: strings: a single CR (carriage return) character, a single LF (line-
! 907: feed) character, the two-character sequence CRLF, any of the three pre-
! 908: ceding, or any Unicode newline sequence. The Unicode newline sequences
! 909: are the three just mentioned, plus the single characters VT (vertical
! 910: tab, U+000B), FF (formfeed, U+000C), NEL (next line, U+0085), LS (line
! 911: separator, U+2028), and PS (paragraph separator, U+2029).
! 912:
! 913: Each of the first three conventions is used by at least one operating
! 914: system as its standard newline sequence. When PCRE is built, a default
! 915: can be specified. The default default is LF, which is the Unix stan-
! 916: dard. When PCRE is run, the default can be overridden, either when a
! 917: pattern is compiled, or when it is matched.
! 918:
! 919: At compile time, the newline convention can be specified by the options
! 920: argument of pcre_compile(), or it can be specified by special text at
! 921: the start of the pattern itself; this overrides any other settings. See
! 922: the pcrepattern page for details of the special character sequences.
! 923:
! 924: In the PCRE documentation the word "newline" is used to mean "the char-
! 925: acter or pair of characters that indicate a line break". The choice of
! 926: newline convention affects the handling of the dot, circumflex, and
! 927: dollar metacharacters, the handling of #-comments in /x mode, and, when
! 928: CRLF is a recognized line ending sequence, the match position advance-
! 929: ment for a non-anchored pattern. There is more detail about this in the
! 930: section on pcre_exec() options below.
! 931:
! 932: The choice of newline convention does not affect the interpretation of
! 933: the \n or \r escape sequences, nor does it affect what \R matches,
! 934: which is controlled in a similar way, but by separate options.
! 935:
! 936:
! 937: MULTITHREADING
! 938:
! 939: The PCRE functions can be used in multi-threading applications, with
! 940: the proviso that the memory management functions pointed to by
! 941: pcre_malloc, pcre_free, pcre_stack_malloc, and pcre_stack_free, and the
! 942: callout function pointed to by pcre_callout, are shared by all threads.
! 943:
! 944: The compiled form of a regular expression is not altered during match-
! 945: ing, so the same compiled pattern can safely be used by several threads
! 946: at once.
! 947:
! 948: If the just-in-time optimization feature is being used, it needs sepa-
! 949: rate memory stack areas for each thread. See the pcrejit documentation
! 950: for more details.
! 951:
! 952:
! 953: SAVING PRECOMPILED PATTERNS FOR LATER USE
! 954:
! 955: The compiled form of a regular expression can be saved and re-used at a
! 956: later time, possibly by a different program, and even on a host other
! 957: than the one on which it was compiled. Details are given in the
! 958: pcreprecompile documentation. However, compiling a regular expression
! 959: with one version of PCRE for use with a different version is not guar-
! 960: anteed to work and may cause crashes.
! 961:
! 962:
! 963: CHECKING BUILD-TIME OPTIONS
! 964:
! 965: int pcre_config(int what, void *where);
! 966:
! 967: The function pcre_config() makes it possible for a PCRE client to dis-
! 968: cover which optional features have been compiled into the PCRE library.
! 969: The pcrebuild documentation has more details about these optional fea-
! 970: tures.
! 971:
! 972: The first argument for pcre_config() is an integer, specifying which
! 973: information is required; the second argument is a pointer to a variable
! 974: into which the information is placed. The following information is
! 975: available:
! 976:
! 977: PCRE_CONFIG_UTF8
! 978:
! 979: The output is an integer that is set to one if UTF-8 support is avail-
! 980: able; otherwise it is set to zero.
! 981:
! 982: PCRE_CONFIG_UNICODE_PROPERTIES
! 983:
! 984: The output is an integer that is set to one if support for Unicode
! 985: character properties is available; otherwise it is set to zero.
! 986:
! 987: PCRE_CONFIG_JIT
! 988:
! 989: The output is an integer that is set to one if support for just-in-time
! 990: compiling is available; otherwise it is set to zero.
! 991:
! 992: PCRE_CONFIG_NEWLINE
! 993:
! 994: The output is an integer whose value specifies the default character
! 995: sequence that is recognized as meaning "newline". The four values that
! 996: are supported are: 10 for LF, 13 for CR, 3338 for CRLF, -2 for ANYCRLF,
! 997: and -1 for ANY. Though they are derived from ASCII, the same values
! 998: are returned in EBCDIC environments. The default should normally corre-
! 999: spond to the standard sequence for your operating system.
! 1000:
! 1001: PCRE_CONFIG_BSR
! 1002:
! 1003: The output is an integer whose value indicates what character sequences
! 1004: the \R escape sequence matches by default. A value of 0 means that \R
! 1005: matches any Unicode line ending sequence; a value of 1 means that \R
! 1006: matches only CR, LF, or CRLF. The default can be overridden when a pat-
! 1007: tern is compiled or matched.
! 1008:
! 1009: PCRE_CONFIG_LINK_SIZE
! 1010:
! 1011: The output is an integer that contains the number of bytes used for
! 1012: internal linkage in compiled regular expressions. The value is 2, 3, or
! 1013: 4. Larger values allow larger regular expressions to be compiled, at
! 1014: the expense of slower matching. The default value of 2 is sufficient
! 1015: for all but the most massive patterns, since it allows the compiled
! 1016: pattern to be up to 64K in size.
! 1017:
! 1018: PCRE_CONFIG_POSIX_MALLOC_THRESHOLD
! 1019:
! 1020: The output is an integer that contains the threshold above which the
! 1021: POSIX interface uses malloc() for output vectors. Further details are
! 1022: given in the pcreposix documentation.
! 1023:
! 1024: PCRE_CONFIG_MATCH_LIMIT
! 1025:
! 1026: The output is a long integer that gives the default limit for the num-
! 1027: ber of internal matching function calls in a pcre_exec() execution.
! 1028: Further details are given with pcre_exec() below.
! 1029:
! 1030: PCRE_CONFIG_MATCH_LIMIT_RECURSION
! 1031:
! 1032: The output is a long integer that gives the default limit for the depth
! 1033: of recursion when calling the internal matching function in a
! 1034: pcre_exec() execution. Further details are given with pcre_exec()
! 1035: below.
! 1036:
! 1037: PCRE_CONFIG_STACKRECURSE
! 1038:
! 1039: The output is an integer that is set to one if internal recursion when
! 1040: running pcre_exec() is implemented by recursive function calls that use
! 1041: the stack to remember their state. This is the usual way that PCRE is
! 1042: compiled. The output is zero if PCRE was compiled to use blocks of data
! 1043: on the heap instead of recursive function calls. In this case,
! 1044: pcre_stack_malloc and pcre_stack_free are called to manage memory
! 1045: blocks on the heap, thus avoiding the use of the stack.
! 1046:
! 1047:
! 1048: COMPILING A PATTERN
! 1049:
! 1050: pcre *pcre_compile(const char *pattern, int options,
! 1051: const char **errptr, int *erroffset,
! 1052: const unsigned char *tableptr);
! 1053:
! 1054: pcre *pcre_compile2(const char *pattern, int options,
! 1055: int *errorcodeptr,
! 1056: const char **errptr, int *erroffset,
! 1057: const unsigned char *tableptr);
! 1058:
! 1059: Either of the functions pcre_compile() or pcre_compile2() can be called
! 1060: to compile a pattern into an internal form. The only difference between
! 1061: the two interfaces is that pcre_compile2() has an additional argument,
! 1062: errorcodeptr, via which a numerical error code can be returned. To
! 1063: avoid too much repetition, we refer just to pcre_compile() below, but
! 1064: the information applies equally to pcre_compile2().
! 1065:
! 1066: The pattern is a C string terminated by a binary zero, and is passed in
! 1067: the pattern argument. A pointer to a single block of memory that is
! 1068: obtained via pcre_malloc is returned. This contains the compiled code
! 1069: and related data. The pcre type is defined for the returned block; this
! 1070: is a typedef for a structure whose contents are not externally defined.
! 1071: It is up to the caller to free the memory (via pcre_free) when it is no
! 1072: longer required.
! 1073:
! 1074: Although the compiled code of a PCRE regex is relocatable, that is, it
! 1075: does not depend on memory location, the complete pcre data block is not
! 1076: fully relocatable, because it may contain a copy of the tableptr argu-
! 1077: ment, which is an address (see below).
! 1078:
! 1079: The options argument contains various bit settings that affect the com-
! 1080: pilation. It should be zero if no options are required. The available
! 1081: options are described below. Some of them (in particular, those that
! 1082: are compatible with Perl, but some others as well) can also be set and
! 1083: unset from within the pattern (see the detailed description in the
! 1084: pcrepattern documentation). For those options that can be different in
! 1085: different parts of the pattern, the contents of the options argument
! 1086: specifies their settings at the start of compilation and execution. The
! 1087: PCRE_ANCHORED, PCRE_BSR_xxx, PCRE_NEWLINE_xxx, PCRE_NO_UTF8_CHECK, and
! 1088: PCRE_NO_START_OPT options can be set at the time of matching as well as
! 1089: at compile time.
! 1090:
! 1091: If errptr is NULL, pcre_compile() returns NULL immediately. Otherwise,
! 1092: if compilation of a pattern fails, pcre_compile() returns NULL, and
! 1093: sets the variable pointed to by errptr to point to a textual error mes-
! 1094: sage. This is a static string that is part of the library. You must not
! 1095: try to free it. Normally, the offset from the start of the pattern to
! 1096: the byte that was being processed when the error was discovered is
! 1097: placed in the variable pointed to by erroffset, which must not be NULL
! 1098: (if it is, an immediate error is given). However, for an invalid UTF-8
! 1099: string, the offset is that of the first byte of the failing character.
! 1100: Also, some errors are not detected until checks are carried out when
! 1101: the whole pattern has been scanned; in these cases the offset passed
! 1102: back is the length of the pattern.
! 1103:
! 1104: Note that the offset is in bytes, not characters, even in UTF-8 mode.
! 1105: It may sometimes point into the middle of a UTF-8 character.
! 1106:
! 1107: If pcre_compile2() is used instead of pcre_compile(), and the error-
! 1108: codeptr argument is not NULL, a non-zero error code number is returned
! 1109: via this argument in the event of an error. This is in addition to the
! 1110: textual error message. Error codes and messages are listed below.
! 1111:
! 1112: If the final argument, tableptr, is NULL, PCRE uses a default set of
! 1113: character tables that are built when PCRE is compiled, using the
! 1114: default C locale. Otherwise, tableptr must be an address that is the
! 1115: result of a call to pcre_maketables(). This value is stored with the
! 1116: compiled pattern, and used again by pcre_exec(), unless another table
! 1117: pointer is passed to it. For more discussion, see the section on locale
! 1118: support below.
! 1119:
! 1120: This code fragment shows a typical straightforward call to pcre_com-
! 1121: pile():
! 1122:
! 1123: pcre *re;
! 1124: const char *error;
! 1125: int erroffset;
! 1126: re = pcre_compile(
! 1127: "^A.*Z", /* the pattern */
! 1128: 0, /* default options */
! 1129: &error, /* for error message */
! 1130: &erroffset, /* for error offset */
! 1131: NULL); /* use default character tables */
! 1132:
! 1133: The following names for option bits are defined in the pcre.h header
! 1134: file:
! 1135:
! 1136: PCRE_ANCHORED
! 1137:
! 1138: If this bit is set, the pattern is forced to be "anchored", that is, it
! 1139: is constrained to match only at the first matching point in the string
! 1140: that is being searched (the "subject string"). This effect can also be
! 1141: achieved by appropriate constructs in the pattern itself, which is the
! 1142: only way to do it in Perl.
! 1143:
! 1144: PCRE_AUTO_CALLOUT
! 1145:
! 1146: If this bit is set, pcre_compile() automatically inserts callout items,
! 1147: all with number 255, before each pattern item. For discussion of the
! 1148: callout facility, see the pcrecallout documentation.
! 1149:
! 1150: PCRE_BSR_ANYCRLF
! 1151: PCRE_BSR_UNICODE
! 1152:
! 1153: These options (which are mutually exclusive) control what the \R escape
! 1154: sequence matches. The choice is either to match only CR, LF, or CRLF,
! 1155: or to match any Unicode newline sequence. The default is specified when
! 1156: PCRE is built. It can be overridden from within the pattern, or by set-
! 1157: ting an option when a compiled pattern is matched.
! 1158:
! 1159: PCRE_CASELESS
! 1160:
! 1161: If this bit is set, letters in the pattern match both upper and lower
! 1162: case letters. It is equivalent to Perl's /i option, and it can be
! 1163: changed within a pattern by a (?i) option setting. In UTF-8 mode, PCRE
! 1164: always understands the concept of case for characters whose values are
! 1165: less than 128, so caseless matching is always possible. For characters
! 1166: with higher values, the concept of case is supported if PCRE is com-
! 1167: piled with Unicode property support, but not otherwise. If you want to
! 1168: use caseless matching for characters 128 and above, you must ensure
! 1169: that PCRE is compiled with Unicode property support as well as with
! 1170: UTF-8 support.
! 1171:
! 1172: PCRE_DOLLAR_ENDONLY
! 1173:
! 1174: If this bit is set, a dollar metacharacter in the pattern matches only
! 1175: at the end of the subject string. Without this option, a dollar also
! 1176: matches immediately before a newline at the end of the string (but not
! 1177: before any other newlines). The PCRE_DOLLAR_ENDONLY option is ignored
! 1178: if PCRE_MULTILINE is set. There is no equivalent to this option in
! 1179: Perl, and no way to set it within a pattern.
! 1180:
! 1181: PCRE_DOTALL
! 1182:
! 1183: If this bit is set, a dot metacharacter in the pattern matches a char-
! 1184: acter of any value, including one that indicates a newline. However, it
! 1185: only ever matches one character, even if newlines are coded as CRLF.
! 1186: Without this option, a dot does not match when the current position is
! 1187: at a newline. This option is equivalent to Perl's /s option, and it can
! 1188: be changed within a pattern by a (?s) option setting. A negative class
! 1189: such as [^a] always matches newline characters, independent of the set-
! 1190: ting of this option.
! 1191:
! 1192: PCRE_DUPNAMES
! 1193:
! 1194: If this bit is set, names used to identify capturing subpatterns need
! 1195: not be unique. This can be helpful for certain types of pattern when it
! 1196: is known that only one instance of the named subpattern can ever be
! 1197: matched. There are more details of named subpatterns below; see also
! 1198: the pcrepattern documentation.
! 1199:
! 1200: PCRE_EXTENDED
! 1201:
! 1202: If this bit is set, whitespace data characters in the pattern are
! 1203: totally ignored except when escaped or inside a character class. White-
! 1204: space does not include the VT character (code 11). In addition, charac-
! 1205: ters between an unescaped # outside a character class and the next new-
! 1206: line, inclusive, are also ignored. This is equivalent to Perl's /x
! 1207: option, and it can be changed within a pattern by a (?x) option set-
! 1208: ting.
! 1209:
! 1210: Which characters are interpreted as newlines is controlled by the
! 1211: options passed to pcre_compile() or by a special sequence at the start
! 1212: of the pattern, as described in the section entitled "Newline conven-
! 1213: tions" in the pcrepattern documentation. Note that the end of this type
! 1214: of comment is a literal newline sequence in the pattern; escape
! 1215: sequences that happen to represent a newline do not count.
! 1216:
! 1217: This option makes it possible to include comments inside complicated
! 1218: patterns. Note, however, that this applies only to data characters.
! 1219: Whitespace characters may never appear within special character
! 1220: sequences in a pattern, for example within the sequence (?( that intro-
! 1221: duces a conditional subpattern.
! 1222:
! 1223: PCRE_EXTRA
! 1224:
! 1225: This option was invented in order to turn on additional functionality
! 1226: of PCRE that is incompatible with Perl, but it is currently of very
! 1227: little use. When set, any backslash in a pattern that is followed by a
! 1228: letter that has no special meaning causes an error, thus reserving
! 1229: these combinations for future expansion. By default, as in Perl, a
! 1230: backslash followed by a letter with no special meaning is treated as a
! 1231: literal. (Perl can, however, be persuaded to give an error for this, by
! 1232: running it with the -w option.) There are at present no other features
! 1233: controlled by this option. It can also be set by a (?X) option setting
! 1234: within a pattern.
! 1235:
! 1236: PCRE_FIRSTLINE
! 1237:
! 1238: If this option is set, an unanchored pattern is required to match
! 1239: before or at the first newline in the subject string, though the
! 1240: matched text may continue over the newline.
! 1241:
! 1242: PCRE_JAVASCRIPT_COMPAT
! 1243:
! 1244: If this option is set, PCRE's behaviour is changed in some ways so that
! 1245: it is compatible with JavaScript rather than Perl. The changes are as
! 1246: follows:
! 1247:
! 1248: (1) A lone closing square bracket in a pattern causes a compile-time
! 1249: error, because this is illegal in JavaScript (by default it is treated
! 1250: as a data character). Thus, the pattern AB]CD becomes illegal when this
! 1251: option is set.
! 1252:
! 1253: (2) At run time, a back reference to an unset subpattern group matches
! 1254: an empty string (by default this causes the current matching alterna-
! 1255: tive to fail). A pattern such as (\1)(a) succeeds when this option is
! 1256: set (assuming it can find an "a" in the subject), whereas it fails by
! 1257: default, for Perl compatibility.
! 1258:
! 1259: (3) \U matches an upper case "U" character; by default \U causes a com-
! 1260: pile time error (Perl uses \U to upper case subsequent characters).
! 1261:
! 1262: (4) \u matches a lower case "u" character unless it is followed by four
! 1263: hexadecimal digits, in which case the hexadecimal number defines the
! 1264: code point to match. By default, \u causes a compile time error (Perl
! 1265: uses it to upper case the following character).
! 1266:
! 1267: (5) \x matches a lower case "x" character unless it is followed by two
! 1268: hexadecimal digits, in which case the hexadecimal number defines the
! 1269: code point to match. By default, as in Perl, a hexadecimal number is
! 1270: always expected after \x, but it may have zero, one, or two digits (so,
! 1271: for example, \xz matches a binary zero character followed by z).
! 1272:
! 1273: PCRE_MULTILINE
! 1274:
! 1275: By default, PCRE treats the subject string as consisting of a single
! 1276: line of characters (even if it actually contains newlines). The "start
! 1277: of line" metacharacter (^) matches only at the start of the string,
! 1278: while the "end of line" metacharacter ($) matches only at the end of
! 1279: the string, or before a terminating newline (unless PCRE_DOLLAR_ENDONLY
! 1280: is set). This is the same as Perl.
! 1281:
! 1282: When PCRE_MULTILINE it is set, the "start of line" and "end of line"
! 1283: constructs match immediately following or immediately before internal
! 1284: newlines in the subject string, respectively, as well as at the very
! 1285: start and end. This is equivalent to Perl's /m option, and it can be
! 1286: changed within a pattern by a (?m) option setting. If there are no new-
! 1287: lines in a subject string, or no occurrences of ^ or $ in a pattern,
! 1288: setting PCRE_MULTILINE has no effect.
! 1289:
! 1290: PCRE_NEWLINE_CR
! 1291: PCRE_NEWLINE_LF
! 1292: PCRE_NEWLINE_CRLF
! 1293: PCRE_NEWLINE_ANYCRLF
! 1294: PCRE_NEWLINE_ANY
! 1295:
! 1296: These options override the default newline definition that was chosen
! 1297: when PCRE was built. Setting the first or the second specifies that a
! 1298: newline is indicated by a single character (CR or LF, respectively).
! 1299: Setting PCRE_NEWLINE_CRLF specifies that a newline is indicated by the
! 1300: two-character CRLF sequence. Setting PCRE_NEWLINE_ANYCRLF specifies
! 1301: that any of the three preceding sequences should be recognized. Setting
! 1302: PCRE_NEWLINE_ANY specifies that any Unicode newline sequence should be
! 1303: recognized. The Unicode newline sequences are the three just mentioned,
! 1304: plus the single characters VT (vertical tab, U+000B), FF (formfeed,
! 1305: U+000C), NEL (next line, U+0085), LS (line separator, U+2028), and PS
! 1306: (paragraph separator, U+2029). The last two are recognized only in
! 1307: UTF-8 mode.
! 1308:
! 1309: The newline setting in the options word uses three bits that are
! 1310: treated as a number, giving eight possibilities. Currently only six are
! 1311: used (default plus the five values above). This means that if you set
! 1312: more than one newline option, the combination may or may not be sensi-
! 1313: ble. For example, PCRE_NEWLINE_CR with PCRE_NEWLINE_LF is equivalent to
! 1314: PCRE_NEWLINE_CRLF, but other combinations may yield unused numbers and
! 1315: cause an error.
! 1316:
! 1317: The only time that a line break in a pattern is specially recognized
! 1318: when compiling is when PCRE_EXTENDED is set. CR and LF are whitespace
! 1319: characters, and so are ignored in this mode. Also, an unescaped # out-
! 1320: side a character class indicates a comment that lasts until after the
! 1321: next line break sequence. In other circumstances, line break sequences
! 1322: in patterns are treated as literal data.
! 1323:
! 1324: The newline option that is set at compile time becomes the default that
! 1325: is used for pcre_exec() and pcre_dfa_exec(), but it can be overridden.
! 1326:
! 1327: PCRE_NO_AUTO_CAPTURE
! 1328:
! 1329: If this option is set, it disables the use of numbered capturing paren-
! 1330: theses in the pattern. Any opening parenthesis that is not followed by
! 1331: ? behaves as if it were followed by ?: but named parentheses can still
! 1332: be used for capturing (and they acquire numbers in the usual way).
! 1333: There is no equivalent of this option in Perl.
! 1334:
! 1335: NO_START_OPTIMIZE
! 1336:
! 1337: This is an option that acts at matching time; that is, it is really an
! 1338: option for pcre_exec() or pcre_dfa_exec(). If it is set at compile
! 1339: time, it is remembered with the compiled pattern and assumed at match-
! 1340: ing time. For details see the discussion of PCRE_NO_START_OPTIMIZE
! 1341: below.
! 1342:
! 1343: PCRE_UCP
! 1344:
! 1345: This option changes the way PCRE processes \B, \b, \D, \d, \S, \s, \W,
! 1346: \w, and some of the POSIX character classes. By default, only ASCII
! 1347: characters are recognized, but if PCRE_UCP is set, Unicode properties
! 1348: are used instead to classify characters. More details are given in the
! 1349: section on generic character types in the pcrepattern page. If you set
! 1350: PCRE_UCP, matching one of the items it affects takes much longer. The
! 1351: option is available only if PCRE has been compiled with Unicode prop-
! 1352: erty support.
! 1353:
! 1354: PCRE_UNGREEDY
! 1355:
! 1356: This option inverts the "greediness" of the quantifiers so that they
! 1357: are not greedy by default, but become greedy if followed by "?". It is
! 1358: not compatible with Perl. It can also be set by a (?U) option setting
! 1359: within the pattern.
! 1360:
! 1361: PCRE_UTF8
! 1362:
! 1363: This option causes PCRE to regard both the pattern and the subject as
! 1364: strings of UTF-8 characters instead of single-byte character strings.
! 1365: However, it is available only when PCRE is built to include UTF-8 sup-
! 1366: port. If not, the use of this option provokes an error. Details of how
! 1367: this option changes the behaviour of PCRE are given in the pcreunicode
! 1368: page.
! 1369:
! 1370: PCRE_NO_UTF8_CHECK
! 1371:
! 1372: When PCRE_UTF8 is set, the validity of the pattern as a UTF-8 string is
! 1373: automatically checked. There is a discussion about the validity of
! 1374: UTF-8 strings in the main pcre page. If an invalid UTF-8 sequence of
! 1375: bytes is found, pcre_compile() returns an error. If you already know
! 1376: that your pattern is valid, and you want to skip this check for perfor-
! 1377: mance reasons, you can set the PCRE_NO_UTF8_CHECK option. When it is
! 1378: set, the effect of passing an invalid UTF-8 string as a pattern is
! 1379: undefined. It may cause your program to crash. Note that this option
! 1380: can also be passed to pcre_exec() and pcre_dfa_exec(), to suppress the
! 1381: UTF-8 validity checking of subject strings.
! 1382:
! 1383:
! 1384: COMPILATION ERROR CODES
! 1385:
! 1386: The following table lists the error codes than may be returned by
! 1387: pcre_compile2(), along with the error messages that may be returned by
! 1388: both compiling functions. As PCRE has developed, some error codes have
! 1389: fallen out of use. To avoid confusion, they have not been re-used.
! 1390:
! 1391: 0 no error
! 1392: 1 \ at end of pattern
! 1393: 2 \c at end of pattern
! 1394: 3 unrecognized character follows \
! 1395: 4 numbers out of order in {} quantifier
! 1396: 5 number too big in {} quantifier
! 1397: 6 missing terminating ] for character class
! 1398: 7 invalid escape sequence in character class
! 1399: 8 range out of order in character class
! 1400: 9 nothing to repeat
! 1401: 10 [this code is not in use]
! 1402: 11 internal error: unexpected repeat
! 1403: 12 unrecognized character after (? or (?-
! 1404: 13 POSIX named classes are supported only within a class
! 1405: 14 missing )
! 1406: 15 reference to non-existent subpattern
! 1407: 16 erroffset passed as NULL
! 1408: 17 unknown option bit(s) set
! 1409: 18 missing ) after comment
! 1410: 19 [this code is not in use]
! 1411: 20 regular expression is too large
! 1412: 21 failed to get memory
! 1413: 22 unmatched parentheses
! 1414: 23 internal error: code overflow
! 1415: 24 unrecognized character after (?<
! 1416: 25 lookbehind assertion is not fixed length
! 1417: 26 malformed number or name after (?(
! 1418: 27 conditional group contains more than two branches
! 1419: 28 assertion expected after (?(
! 1420: 29 (?R or (?[+-]digits must be followed by )
! 1421: 30 unknown POSIX class name
! 1422: 31 POSIX collating elements are not supported
! 1423: 32 this version of PCRE is not compiled with PCRE_UTF8 support
! 1424: 33 [this code is not in use]
! 1425: 34 character value in \x{...} sequence is too large
! 1426: 35 invalid condition (?(0)
! 1427: 36 \C not allowed in lookbehind assertion
! 1428: 37 PCRE does not support \L, \l, \N{name}, \U, or \u
! 1429: 38 number after (?C is > 255
! 1430: 39 closing ) for (?C expected
! 1431: 40 recursive call could loop indefinitely
! 1432: 41 unrecognized character after (?P
! 1433: 42 syntax error in subpattern name (missing terminator)
! 1434: 43 two named subpatterns have the same name
! 1435: 44 invalid UTF-8 string
! 1436: 45 support for \P, \p, and \X has not been compiled
! 1437: 46 malformed \P or \p sequence
! 1438: 47 unknown property name after \P or \p
! 1439: 48 subpattern name is too long (maximum 32 characters)
! 1440: 49 too many named subpatterns (maximum 10000)
! 1441: 50 [this code is not in use]
! 1442: 51 octal value is greater than \377 (not in UTF-8 mode)
! 1443: 52 internal error: overran compiling workspace
! 1444: 53 internal error: previously-checked referenced subpattern
! 1445: not found
! 1446: 54 DEFINE group contains more than one branch
! 1447: 55 repeating a DEFINE group is not allowed
! 1448: 56 inconsistent NEWLINE options
! 1449: 57 \g is not followed by a braced, angle-bracketed, or quoted
! 1450: name/number or by a plain number
! 1451: 58 a numbered reference must not be zero
! 1452: 59 an argument is not allowed for (*ACCEPT), (*FAIL), or (*COMMIT)
! 1453: 60 (*VERB) not recognized
! 1454: 61 number is too big
! 1455: 62 subpattern name expected
! 1456: 63 digit expected after (?+
! 1457: 64 ] is an invalid data character in JavaScript compatibility mode
! 1458: 65 different names for subpatterns of the same number are
! 1459: not allowed
! 1460: 66 (*MARK) must have an argument
! 1461: 67 this version of PCRE is not compiled with PCRE_UCP support
! 1462: 68 \c must be followed by an ASCII character
! 1463: 69 \k is not followed by a braced, angle-bracketed, or quoted name
! 1464:
! 1465: The numbers 32 and 10000 in errors 48 and 49 are defaults; different
! 1466: values may be used if the limits were changed when PCRE was built.
! 1467:
! 1468:
! 1469: STUDYING A PATTERN
! 1470:
! 1471: pcre_extra *pcre_study(const pcre *code, int options
! 1472: const char **errptr);
! 1473:
! 1474: If a compiled pattern is going to be used several times, it is worth
! 1475: spending more time analyzing it in order to speed up the time taken for
! 1476: matching. The function pcre_study() takes a pointer to a compiled pat-
! 1477: tern as its first argument. If studying the pattern produces additional
! 1478: information that will help speed up matching, pcre_study() returns a
! 1479: pointer to a pcre_extra block, in which the study_data field points to
! 1480: the results of the study.
! 1481:
! 1482: The returned value from pcre_study() can be passed directly to
! 1483: pcre_exec() or pcre_dfa_exec(). However, a pcre_extra block also con-
! 1484: tains other fields that can be set by the caller before the block is
! 1485: passed; these are described below in the section on matching a pattern.
! 1486:
! 1487: If studying the pattern does not produce any useful information,
! 1488: pcre_study() returns NULL. In that circumstance, if the calling program
! 1489: wants to pass any of the other fields to pcre_exec() or
! 1490: pcre_dfa_exec(), it must set up its own pcre_extra block.
! 1491:
! 1492: The second argument of pcre_study() contains option bits. There is only
! 1493: one option: PCRE_STUDY_JIT_COMPILE. If this is set, and the just-in-
! 1494: time compiler is available, the pattern is further compiled into
! 1495: machine code that executes much faster than the pcre_exec() matching
! 1496: function. If the just-in-time compiler is not available, this option is
! 1497: ignored. All other bits in the options argument must be zero.
! 1498:
! 1499: JIT compilation is a heavyweight optimization. It can take some time
! 1500: for patterns to be analyzed, and for one-off matches and simple pat-
! 1501: terns the benefit of faster execution might be offset by a much slower
! 1502: study time. Not all patterns can be optimized by the JIT compiler. For
! 1503: those that cannot be handled, matching automatically falls back to the
! 1504: pcre_exec() interpreter. For more details, see the pcrejit documenta-
! 1505: tion.
! 1506:
! 1507: The third argument for pcre_study() is a pointer for an error message.
! 1508: If studying succeeds (even if no data is returned), the variable it
! 1509: points to is set to NULL. Otherwise it is set to point to a textual
! 1510: error message. This is a static string that is part of the library. You
! 1511: must not try to free it. You should test the error pointer for NULL
! 1512: after calling pcre_study(), to be sure that it has run successfully.
! 1513:
! 1514: When you are finished with a pattern, you can free the memory used for
! 1515: the study data by calling pcre_free_study(). This function was added to
! 1516: the API for release 8.20. For earlier versions, the memory could be
! 1517: freed with pcre_free(), just like the pattern itself. This will still
! 1518: work in cases where PCRE_STUDY_JIT_COMPILE is not used, but it is
! 1519: advisable to change to the new function when convenient.
! 1520:
! 1521: This is a typical way in which pcre_study() is used (except that in a
! 1522: real application there should be tests for errors):
! 1523:
! 1524: int rc;
! 1525: pcre *re;
! 1526: pcre_extra *sd;
! 1527: re = pcre_compile("pattern", 0, &error, &erroroffset, NULL);
! 1528: sd = pcre_study(
! 1529: re, /* result of pcre_compile() */
! 1530: 0, /* no options */
! 1531: &error); /* set to NULL or points to a message */
! 1532: rc = pcre_exec( /* see below for details of pcre_exec() options */
! 1533: re, sd, "subject", 7, 0, 0, ovector, 30);
! 1534: ...
! 1535: pcre_free_study(sd);
! 1536: pcre_free(re);
! 1537:
! 1538: Studying a pattern does two things: first, a lower bound for the length
! 1539: of subject string that is needed to match the pattern is computed. This
! 1540: does not mean that there are any strings of that length that match, but
! 1541: it does guarantee that no shorter strings match. The value is used by
! 1542: pcre_exec() and pcre_dfa_exec() to avoid wasting time by trying to
! 1543: match strings that are shorter than the lower bound. You can find out
! 1544: the value in a calling program via the pcre_fullinfo() function.
! 1545:
! 1546: Studying a pattern is also useful for non-anchored patterns that do not
! 1547: have a single fixed starting character. A bitmap of possible starting
! 1548: bytes is created. This speeds up finding a position in the subject at
! 1549: which to start matching.
! 1550:
! 1551: These two optimizations apply to both pcre_exec() and pcre_dfa_exec().
! 1552: However, they are not used by pcre_exec() if pcre_study() is called
! 1553: with the PCRE_STUDY_JIT_COMPILE option, and just-in-time compiling is
! 1554: successful. The optimizations can be disabled by setting the
! 1555: PCRE_NO_START_OPTIMIZE option when calling pcre_exec() or
! 1556: pcre_dfa_exec(). You might want to do this if your pattern contains
! 1557: callouts or (*MARK) (which cannot be handled by the JIT compiler), and
! 1558: you want to make use of these facilities in cases where matching fails.
! 1559: See the discussion of PCRE_NO_START_OPTIMIZE below.
! 1560:
! 1561:
! 1562: LOCALE SUPPORT
! 1563:
! 1564: PCRE handles caseless matching, and determines whether characters are
! 1565: letters, digits, or whatever, by reference to a set of tables, indexed
! 1566: by character value. When running in UTF-8 mode, this applies only to
! 1567: characters with codes less than 128. By default, higher-valued codes
! 1568: never match escapes such as \w or \d, but they can be tested with \p if
! 1569: PCRE is built with Unicode character property support. Alternatively,
! 1570: the PCRE_UCP option can be set at compile time; this causes \w and
! 1571: friends to use Unicode property support instead of built-in tables. The
! 1572: use of locales with Unicode is discouraged. If you are handling charac-
! 1573: ters with codes greater than 128, you should either use UTF-8 and Uni-
! 1574: code, or use locales, but not try to mix the two.
! 1575:
! 1576: PCRE contains an internal set of tables that are used when the final
! 1577: argument of pcre_compile() is NULL. These are sufficient for many
! 1578: applications. Normally, the internal tables recognize only ASCII char-
! 1579: acters. However, when PCRE is built, it is possible to cause the inter-
! 1580: nal tables to be rebuilt in the default "C" locale of the local system,
! 1581: which may cause them to be different.
! 1582:
! 1583: The internal tables can always be overridden by tables supplied by the
! 1584: application that calls PCRE. These may be created in a different locale
! 1585: from the default. As more and more applications change to using Uni-
! 1586: code, the need for this locale support is expected to die away.
! 1587:
! 1588: External tables are built by calling the pcre_maketables() function,
! 1589: which has no arguments, in the relevant locale. The result can then be
! 1590: passed to pcre_compile() or pcre_exec() as often as necessary. For
! 1591: example, to build and use tables that are appropriate for the French
! 1592: locale (where accented characters with values greater than 128 are
! 1593: treated as letters), the following code could be used:
! 1594:
! 1595: setlocale(LC_CTYPE, "fr_FR");
! 1596: tables = pcre_maketables();
! 1597: re = pcre_compile(..., tables);
! 1598:
! 1599: The locale name "fr_FR" is used on Linux and other Unix-like systems;
! 1600: if you are using Windows, the name for the French locale is "french".
! 1601:
! 1602: When pcre_maketables() runs, the tables are built in memory that is
! 1603: obtained via pcre_malloc. It is the caller's responsibility to ensure
! 1604: that the memory containing the tables remains available for as long as
! 1605: it is needed.
! 1606:
! 1607: The pointer that is passed to pcre_compile() is saved with the compiled
! 1608: pattern, and the same tables are used via this pointer by pcre_study()
! 1609: and normally also by pcre_exec(). Thus, by default, for any single pat-
! 1610: tern, compilation, studying and matching all happen in the same locale,
! 1611: but different patterns can be compiled in different locales.
! 1612:
! 1613: It is possible to pass a table pointer or NULL (indicating the use of
! 1614: the internal tables) to pcre_exec(). Although not intended for this
! 1615: purpose, this facility could be used to match a pattern in a different
! 1616: locale from the one in which it was compiled. Passing table pointers at
! 1617: run time is discussed below in the section on matching a pattern.
! 1618:
! 1619:
! 1620: INFORMATION ABOUT A PATTERN
! 1621:
! 1622: int pcre_fullinfo(const pcre *code, const pcre_extra *extra,
! 1623: int what, void *where);
! 1624:
! 1625: The pcre_fullinfo() function returns information about a compiled pat-
! 1626: tern. It replaces the obsolete pcre_info() function, which is neverthe-
! 1627: less retained for backwards compability (and is documented below).
! 1628:
! 1629: The first argument for pcre_fullinfo() is a pointer to the compiled
! 1630: pattern. The second argument is the result of pcre_study(), or NULL if
! 1631: the pattern was not studied. The third argument specifies which piece
! 1632: of information is required, and the fourth argument is a pointer to a
! 1633: variable to receive the data. The yield of the function is zero for
! 1634: success, or one of the following negative numbers:
! 1635:
! 1636: PCRE_ERROR_NULL the argument code was NULL
! 1637: the argument where was NULL
! 1638: PCRE_ERROR_BADMAGIC the "magic number" was not found
! 1639: PCRE_ERROR_BADOPTION the value of what was invalid
! 1640:
! 1641: The "magic number" is placed at the start of each compiled pattern as
! 1642: an simple check against passing an arbitrary memory pointer. Here is a
! 1643: typical call of pcre_fullinfo(), to obtain the length of the compiled
! 1644: pattern:
! 1645:
! 1646: int rc;
! 1647: size_t length;
! 1648: rc = pcre_fullinfo(
! 1649: re, /* result of pcre_compile() */
! 1650: sd, /* result of pcre_study(), or NULL */
! 1651: PCRE_INFO_SIZE, /* what is required */
! 1652: &length); /* where to put the data */
! 1653:
! 1654: The possible values for the third argument are defined in pcre.h, and
! 1655: are as follows:
! 1656:
! 1657: PCRE_INFO_BACKREFMAX
! 1658:
! 1659: Return the number of the highest back reference in the pattern. The
! 1660: fourth argument should point to an int variable. Zero is returned if
! 1661: there are no back references.
! 1662:
! 1663: PCRE_INFO_CAPTURECOUNT
! 1664:
! 1665: Return the number of capturing subpatterns in the pattern. The fourth
! 1666: argument should point to an int variable.
! 1667:
! 1668: PCRE_INFO_DEFAULT_TABLES
! 1669:
! 1670: Return a pointer to the internal default character tables within PCRE.
! 1671: The fourth argument should point to an unsigned char * variable. This
! 1672: information call is provided for internal use by the pcre_study() func-
! 1673: tion. External callers can cause PCRE to use its internal tables by
! 1674: passing a NULL table pointer.
! 1675:
! 1676: PCRE_INFO_FIRSTBYTE
! 1677:
! 1678: Return information about the first byte of any matched string, for a
! 1679: non-anchored pattern. The fourth argument should point to an int vari-
! 1680: able. (This option used to be called PCRE_INFO_FIRSTCHAR; the old name
! 1681: is still recognized for backwards compatibility.)
! 1682:
! 1683: If there is a fixed first byte, for example, from a pattern such as
! 1684: (cat|cow|coyote), its value is returned. Otherwise, if either
! 1685:
! 1686: (a) the pattern was compiled with the PCRE_MULTILINE option, and every
! 1687: branch starts with "^", or
! 1688:
! 1689: (b) every branch of the pattern starts with ".*" and PCRE_DOTALL is not
! 1690: set (if it were set, the pattern would be anchored),
! 1691:
! 1692: -1 is returned, indicating that the pattern matches only at the start
! 1693: of a subject string or after any newline within the string. Otherwise
! 1694: -2 is returned. For anchored patterns, -2 is returned.
! 1695:
! 1696: PCRE_INFO_FIRSTTABLE
! 1697:
! 1698: If the pattern was studied, and this resulted in the construction of a
! 1699: 256-bit table indicating a fixed set of bytes for the first byte in any
! 1700: matching string, a pointer to the table is returned. Otherwise NULL is
! 1701: returned. The fourth argument should point to an unsigned char * vari-
! 1702: able.
! 1703:
! 1704: PCRE_INFO_HASCRORLF
! 1705:
! 1706: Return 1 if the pattern contains any explicit matches for CR or LF
! 1707: characters, otherwise 0. The fourth argument should point to an int
! 1708: variable. An explicit match is either a literal CR or LF character, or
! 1709: \r or \n.
! 1710:
! 1711: PCRE_INFO_JCHANGED
! 1712:
! 1713: Return 1 if the (?J) or (?-J) option setting is used in the pattern,
! 1714: otherwise 0. The fourth argument should point to an int variable. (?J)
! 1715: and (?-J) set and unset the local PCRE_DUPNAMES option, respectively.
! 1716:
! 1717: PCRE_INFO_JIT
! 1718:
! 1719: Return 1 if the pattern was studied with the PCRE_STUDY_JIT_COMPILE
! 1720: option, and just-in-time compiling was successful. The fourth argument
! 1721: should point to an int variable. A return value of 0 means that JIT
! 1722: support is not available in this version of PCRE, or that the pattern
! 1723: was not studied with the PCRE_STUDY_JIT_COMPILE option, or that the JIT
! 1724: compiler could not handle this particular pattern. See the pcrejit doc-
! 1725: umentation for details of what can and cannot be handled.
! 1726:
! 1727: PCRE_INFO_JITSIZE
! 1728:
! 1729: If the pattern was successfully studied with the PCRE_STUDY_JIT_COMPILE
! 1730: option, return the size of the JIT compiled code, otherwise return
! 1731: zero. The fourth argument should point to a size_t variable.
! 1732:
! 1733: PCRE_INFO_LASTLITERAL
! 1734:
! 1735: Return the value of the rightmost literal byte that must exist in any
! 1736: matched string, other than at its start, if such a byte has been
! 1737: recorded. The fourth argument should point to an int variable. If there
! 1738: is no such byte, -1 is returned. For anchored patterns, a last literal
! 1739: byte is recorded only if it follows something of variable length. For
! 1740: example, for the pattern /^a\d+z\d+/ the returned value is "z", but for
! 1741: /^a\dz\d/ the returned value is -1.
! 1742:
! 1743: PCRE_INFO_MINLENGTH
! 1744:
! 1745: If the pattern was studied and a minimum length for matching subject
! 1746: strings was computed, its value is returned. Otherwise the returned
! 1747: value is -1. The value is a number of characters, not bytes (this may
! 1748: be relevant in UTF-8 mode). The fourth argument should point to an int
! 1749: variable. A non-negative value is a lower bound to the length of any
! 1750: matching string. There may not be any strings of that length that do
! 1751: actually match, but every string that does match is at least that long.
! 1752:
! 1753: PCRE_INFO_NAMECOUNT
! 1754: PCRE_INFO_NAMEENTRYSIZE
! 1755: PCRE_INFO_NAMETABLE
! 1756:
! 1757: PCRE supports the use of named as well as numbered capturing parenthe-
! 1758: ses. The names are just an additional way of identifying the parenthe-
! 1759: ses, which still acquire numbers. Several convenience functions such as
! 1760: pcre_get_named_substring() are provided for extracting captured sub-
! 1761: strings by name. It is also possible to extract the data directly, by
! 1762: first converting the name to a number in order to access the correct
! 1763: pointers in the output vector (described with pcre_exec() below). To do
! 1764: the conversion, you need to use the name-to-number map, which is
! 1765: described by these three values.
! 1766:
! 1767: The map consists of a number of fixed-size entries. PCRE_INFO_NAMECOUNT
! 1768: gives the number of entries, and PCRE_INFO_NAMEENTRYSIZE gives the size
! 1769: of each entry; both of these return an int value. The entry size
! 1770: depends on the length of the longest name. PCRE_INFO_NAMETABLE returns
! 1771: a pointer to the first entry of the table (a pointer to char). The
! 1772: first two bytes of each entry are the number of the capturing parenthe-
! 1773: sis, most significant byte first. The rest of the entry is the corre-
! 1774: sponding name, zero terminated.
! 1775:
! 1776: The names are in alphabetical order. Duplicate names may appear if (?|
! 1777: is used to create multiple groups with the same number, as described in
! 1778: the section on duplicate subpattern numbers in the pcrepattern page.
! 1779: Duplicate names for subpatterns with different numbers are permitted
! 1780: only if PCRE_DUPNAMES is set. In all cases of duplicate names, they
! 1781: appear in the table in the order in which they were found in the pat-
! 1782: tern. In the absence of (?| this is the order of increasing number;
! 1783: when (?| is used this is not necessarily the case because later subpat-
! 1784: terns may have lower numbers.
! 1785:
! 1786: As a simple example of the name/number table, consider the following
! 1787: pattern (assume PCRE_EXTENDED is set, so white space - including new-
! 1788: lines - is ignored):
! 1789:
! 1790: (?<date> (?<year>(\d\d)?\d\d) -
! 1791: (?<month>\d\d) - (?<day>\d\d) )
! 1792:
! 1793: There are four named subpatterns, so the table has four entries, and
! 1794: each entry in the table is eight bytes long. The table is as follows,
! 1795: with non-printing bytes shows in hexadecimal, and undefined bytes shown
! 1796: as ??:
! 1797:
! 1798: 00 01 d a t e 00 ??
! 1799: 00 05 d a y 00 ?? ??
! 1800: 00 04 m o n t h 00
! 1801: 00 02 y e a r 00 ??
! 1802:
! 1803: When writing code to extract data from named subpatterns using the
! 1804: name-to-number map, remember that the length of the entries is likely
! 1805: to be different for each compiled pattern.
! 1806:
! 1807: PCRE_INFO_OKPARTIAL
! 1808:
! 1809: Return 1 if the pattern can be used for partial matching with
! 1810: pcre_exec(), otherwise 0. The fourth argument should point to an int
! 1811: variable. From release 8.00, this always returns 1, because the
! 1812: restrictions that previously applied to partial matching have been
! 1813: lifted. The pcrepartial documentation gives details of partial match-
! 1814: ing.
! 1815:
! 1816: PCRE_INFO_OPTIONS
! 1817:
! 1818: Return a copy of the options with which the pattern was compiled. The
! 1819: fourth argument should point to an unsigned long int variable. These
! 1820: option bits are those specified in the call to pcre_compile(), modified
! 1821: by any top-level option settings at the start of the pattern itself. In
! 1822: other words, they are the options that will be in force when matching
! 1823: starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with
! 1824: the PCRE_EXTENDED option, the result is PCRE_CASELESS, PCRE_MULTILINE,
! 1825: and PCRE_EXTENDED.
! 1826:
! 1827: A pattern is automatically anchored by PCRE if all of its top-level
! 1828: alternatives begin with one of the following:
! 1829:
! 1830: ^ unless PCRE_MULTILINE is set
! 1831: \A always
! 1832: \G always
! 1833: .* if PCRE_DOTALL is set and there are no back
! 1834: references to the subpattern in which .* appears
! 1835:
! 1836: For such patterns, the PCRE_ANCHORED bit is set in the options returned
! 1837: by pcre_fullinfo().
! 1838:
! 1839: PCRE_INFO_SIZE
! 1840:
! 1841: Return the size of the compiled pattern. The fourth argument should
! 1842: point to a size_t variable. This value does not include the size of the
! 1843: pcre structure that is returned by pcre_compile(). The value that is
! 1844: passed as the argument to pcre_malloc() when pcre_compile() is getting
! 1845: memory in which to place the compiled data is the value returned by
! 1846: this option plus the size of the pcre structure. Studying a compiled
! 1847: pattern, with or without JIT, does not alter the value returned by this
! 1848: option.
! 1849:
! 1850: PCRE_INFO_STUDYSIZE
! 1851:
! 1852: Return the size of the data block pointed to by the study_data field in
! 1853: a pcre_extra block. If pcre_extra is NULL, or there is no study data,
! 1854: zero is returned. The fourth argument should point to a size_t vari-
! 1855: able. The study_data field is set by pcre_study() to record informa-
! 1856: tion that will speed up matching (see the section entitled "Studying a
! 1857: pattern" above). The format of the study_data block is private, but its
! 1858: length is made available via this option so that it can be saved and
! 1859: restored (see the pcreprecompile documentation for details).
! 1860:
! 1861:
! 1862: OBSOLETE INFO FUNCTION
! 1863:
! 1864: int pcre_info(const pcre *code, int *optptr, int *firstcharptr);
! 1865:
! 1866: The pcre_info() function is now obsolete because its interface is too
! 1867: restrictive to return all the available data about a compiled pattern.
! 1868: New programs should use pcre_fullinfo() instead. The yield of
! 1869: pcre_info() is the number of capturing subpatterns, or one of the fol-
! 1870: lowing negative numbers:
! 1871:
! 1872: PCRE_ERROR_NULL the argument code was NULL
! 1873: PCRE_ERROR_BADMAGIC the "magic number" was not found
! 1874:
! 1875: If the optptr argument is not NULL, a copy of the options with which
! 1876: the pattern was compiled is placed in the integer it points to (see
! 1877: PCRE_INFO_OPTIONS above).
! 1878:
! 1879: If the pattern is not anchored and the firstcharptr argument is not
! 1880: NULL, it is used to pass back information about the first character of
! 1881: any matched string (see PCRE_INFO_FIRSTBYTE above).
! 1882:
! 1883:
! 1884: REFERENCE COUNTS
! 1885:
! 1886: int pcre_refcount(pcre *code, int adjust);
! 1887:
! 1888: The pcre_refcount() function is used to maintain a reference count in
! 1889: the data block that contains a compiled pattern. It is provided for the
! 1890: benefit of applications that operate in an object-oriented manner,
! 1891: where different parts of the application may be using the same compiled
! 1892: pattern, but you want to free the block when they are all done.
! 1893:
! 1894: When a pattern is compiled, the reference count field is initialized to
! 1895: zero. It is changed only by calling this function, whose action is to
! 1896: add the adjust value (which may be positive or negative) to it. The
! 1897: yield of the function is the new value. However, the value of the count
! 1898: is constrained to lie between 0 and 65535, inclusive. If the new value
! 1899: is outside these limits, it is forced to the appropriate limit value.
! 1900:
! 1901: Except when it is zero, the reference count is not correctly preserved
! 1902: if a pattern is compiled on one host and then transferred to a host
! 1903: whose byte-order is different. (This seems a highly unlikely scenario.)
! 1904:
! 1905:
! 1906: MATCHING A PATTERN: THE TRADITIONAL FUNCTION
! 1907:
! 1908: int pcre_exec(const pcre *code, const pcre_extra *extra,
! 1909: const char *subject, int length, int startoffset,
! 1910: int options, int *ovector, int ovecsize);
! 1911:
! 1912: The function pcre_exec() is called to match a subject string against a
! 1913: compiled pattern, which is passed in the code argument. If the pattern
! 1914: was studied, the result of the study should be passed in the extra
! 1915: argument. You can call pcre_exec() with the same code and extra argu-
! 1916: ments as many times as you like, in order to match different subject
! 1917: strings with the same pattern.
! 1918:
! 1919: This function is the main matching facility of the library, and it
! 1920: operates in a Perl-like manner. For specialist use there is also an
! 1921: alternative matching function, which is described below in the section
! 1922: about the pcre_dfa_exec() function.
! 1923:
! 1924: In most applications, the pattern will have been compiled (and option-
! 1925: ally studied) in the same process that calls pcre_exec(). However, it
! 1926: is possible to save compiled patterns and study data, and then use them
! 1927: later in different processes, possibly even on different hosts. For a
! 1928: discussion about this, see the pcreprecompile documentation.
! 1929:
! 1930: Here is an example of a simple call to pcre_exec():
! 1931:
! 1932: int rc;
! 1933: int ovector[30];
! 1934: rc = pcre_exec(
! 1935: re, /* result of pcre_compile() */
! 1936: NULL, /* we didn't study the pattern */
! 1937: "some string", /* the subject string */
! 1938: 11, /* the length of the subject string */
! 1939: 0, /* start at offset 0 in the subject */
! 1940: 0, /* default options */
! 1941: ovector, /* vector of integers for substring information */
! 1942: 30); /* number of elements (NOT size in bytes) */
! 1943:
! 1944: Extra data for pcre_exec()
! 1945:
! 1946: If the extra argument is not NULL, it must point to a pcre_extra data
! 1947: block. The pcre_study() function returns such a block (when it doesn't
! 1948: return NULL), but you can also create one for yourself, and pass addi-
! 1949: tional information in it. The pcre_extra block contains the following
! 1950: fields (not necessarily in this order):
! 1951:
! 1952: unsigned long int flags;
! 1953: void *study_data;
! 1954: void *executable_jit;
! 1955: unsigned long int match_limit;
! 1956: unsigned long int match_limit_recursion;
! 1957: void *callout_data;
! 1958: const unsigned char *tables;
! 1959: unsigned char **mark;
! 1960:
! 1961: The flags field is a bitmap that specifies which of the other fields
! 1962: are set. The flag bits are:
! 1963:
! 1964: PCRE_EXTRA_STUDY_DATA
! 1965: PCRE_EXTRA_EXECUTABLE_JIT
! 1966: PCRE_EXTRA_MATCH_LIMIT
! 1967: PCRE_EXTRA_MATCH_LIMIT_RECURSION
! 1968: PCRE_EXTRA_CALLOUT_DATA
! 1969: PCRE_EXTRA_TABLES
! 1970: PCRE_EXTRA_MARK
! 1971:
! 1972: Other flag bits should be set to zero. The study_data field and some-
! 1973: times the executable_jit field are set in the pcre_extra block that is
! 1974: returned by pcre_study(), together with the appropriate flag bits. You
! 1975: should not set these yourself, but you may add to the block by setting
! 1976: the other fields and their corresponding flag bits.
! 1977:
! 1978: The match_limit field provides a means of preventing PCRE from using up
! 1979: a vast amount of resources when running patterns that are not going to
! 1980: match, but which have a very large number of possibilities in their
! 1981: search trees. The classic example is a pattern that uses nested unlim-
! 1982: ited repeats.
! 1983:
! 1984: Internally, pcre_exec() uses a function called match(), which it calls
! 1985: repeatedly (sometimes recursively). The limit set by match_limit is
! 1986: imposed on the number of times this function is called during a match,
! 1987: which has the effect of limiting the amount of backtracking that can
! 1988: take place. For patterns that are not anchored, the count restarts from
! 1989: zero for each position in the subject string.
! 1990:
! 1991: When pcre_exec() is called with a pattern that was successfully studied
! 1992: with the PCRE_STUDY_JIT_COMPILE option, the way that the matching is
! 1993: executed is entirely different. However, there is still the possibility
! 1994: of runaway matching that goes on for a very long time, and so the
! 1995: match_limit value is also used in this case (but in a different way) to
! 1996: limit how long the matching can continue.
! 1997:
! 1998: The default value for the limit can be set when PCRE is built; the
! 1999: default default is 10 million, which handles all but the most extreme
! 2000: cases. You can override the default by suppling pcre_exec() with a
! 2001: pcre_extra block in which match_limit is set, and
! 2002: PCRE_EXTRA_MATCH_LIMIT is set in the flags field. If the limit is
! 2003: exceeded, pcre_exec() returns PCRE_ERROR_MATCHLIMIT.
! 2004:
! 2005: The match_limit_recursion field is similar to match_limit, but instead
! 2006: of limiting the total number of times that match() is called, it limits
! 2007: the depth of recursion. The recursion depth is a smaller number than
! 2008: the total number of calls, because not all calls to match() are recur-
! 2009: sive. This limit is of use only if it is set smaller than match_limit.
! 2010:
! 2011: Limiting the recursion depth limits the amount of machine stack that
! 2012: can be used, or, when PCRE has been compiled to use memory on the heap
! 2013: instead of the stack, the amount of heap memory that can be used. This
! 2014: limit is not relevant, and is ignored, if the pattern was successfully
! 2015: studied with PCRE_STUDY_JIT_COMPILE.
! 2016:
! 2017: The default value for match_limit_recursion can be set when PCRE is
! 2018: built; the default default is the same value as the default for
! 2019: match_limit. You can override the default by suppling pcre_exec() with
! 2020: a pcre_extra block in which match_limit_recursion is set, and
! 2021: PCRE_EXTRA_MATCH_LIMIT_RECURSION is set in the flags field. If the
! 2022: limit is exceeded, pcre_exec() returns PCRE_ERROR_RECURSIONLIMIT.
! 2023:
! 2024: The callout_data field is used in conjunction with the "callout" fea-
! 2025: ture, and is described in the pcrecallout documentation.
! 2026:
! 2027: The tables field is used to pass a character tables pointer to
! 2028: pcre_exec(); this overrides the value that is stored with the compiled
! 2029: pattern. A non-NULL value is stored with the compiled pattern only if
! 2030: custom tables were supplied to pcre_compile() via its tableptr argu-
! 2031: ment. If NULL is passed to pcre_exec() using this mechanism, it forces
! 2032: PCRE's internal tables to be used. This facility is helpful when re-
! 2033: using patterns that have been saved after compiling with an external
! 2034: set of tables, because the external tables might be at a different
! 2035: address when pcre_exec() is called. See the pcreprecompile documenta-
! 2036: tion for a discussion of saving compiled patterns for later use.
! 2037:
! 2038: If PCRE_EXTRA_MARK is set in the flags field, the mark field must be
! 2039: set to point to a char * variable. If the pattern contains any back-
! 2040: tracking control verbs such as (*MARK:NAME), and the execution ends up
! 2041: with a name to pass back, a pointer to the name string (zero termi-
! 2042: nated) is placed in the variable pointed to by the mark field. The
! 2043: names are within the compiled pattern; if you wish to retain such a
! 2044: name you must copy it before freeing the memory of a compiled pattern.
! 2045: If there is no name to pass back, the variable pointed to by the mark
! 2046: field set to NULL. For details of the backtracking control verbs, see
! 2047: the section entitled "Backtracking control" in the pcrepattern documen-
! 2048: tation.
! 2049:
! 2050: Option bits for pcre_exec()
! 2051:
! 2052: The unused bits of the options argument for pcre_exec() must be zero.
! 2053: The only bits that may be set are PCRE_ANCHORED, PCRE_NEWLINE_xxx,
! 2054: PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
! 2055: PCRE_NO_START_OPTIMIZE, PCRE_NO_UTF8_CHECK, PCRE_PARTIAL_SOFT, and
! 2056: PCRE_PARTIAL_HARD.
! 2057:
! 2058: If the pattern was successfully studied with the PCRE_STUDY_JIT_COMPILE
! 2059: option, the only supported options for JIT execution are
! 2060: PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and
! 2061: PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
! 2062: supported. If an unsupported option is used, JIT execution is disabled
! 2063: and the normal interpretive code in pcre_exec() is run.
! 2064:
! 2065: PCRE_ANCHORED
! 2066:
! 2067: The PCRE_ANCHORED option limits pcre_exec() to matching at the first
! 2068: matching position. If a pattern was compiled with PCRE_ANCHORED, or
! 2069: turned out to be anchored by virtue of its contents, it cannot be made
! 2070: unachored at matching time.
! 2071:
! 2072: PCRE_BSR_ANYCRLF
! 2073: PCRE_BSR_UNICODE
! 2074:
! 2075: These options (which are mutually exclusive) control what the \R escape
! 2076: sequence matches. The choice is either to match only CR, LF, or CRLF,
! 2077: or to match any Unicode newline sequence. These options override the
! 2078: choice that was made or defaulted when the pattern was compiled.
! 2079:
! 2080: PCRE_NEWLINE_CR
! 2081: PCRE_NEWLINE_LF
! 2082: PCRE_NEWLINE_CRLF
! 2083: PCRE_NEWLINE_ANYCRLF
! 2084: PCRE_NEWLINE_ANY
! 2085:
! 2086: These options override the newline definition that was chosen or
! 2087: defaulted when the pattern was compiled. For details, see the descrip-
! 2088: tion of pcre_compile() above. During matching, the newline choice
! 2089: affects the behaviour of the dot, circumflex, and dollar metacharac-
! 2090: ters. It may also alter the way the match position is advanced after a
! 2091: match failure for an unanchored pattern.
! 2092:
! 2093: When PCRE_NEWLINE_CRLF, PCRE_NEWLINE_ANYCRLF, or PCRE_NEWLINE_ANY is
! 2094: set, and a match attempt for an unanchored pattern fails when the cur-
! 2095: rent position is at a CRLF sequence, and the pattern contains no
! 2096: explicit matches for CR or LF characters, the match position is
! 2097: advanced by two characters instead of one, in other words, to after the
! 2098: CRLF.
! 2099:
! 2100: The above rule is a compromise that makes the most common cases work as
! 2101: expected. For example, if the pattern is .+A (and the PCRE_DOTALL
! 2102: option is not set), it does not match the string "\r\nA" because, after
! 2103: failing at the start, it skips both the CR and the LF before retrying.
! 2104: However, the pattern [\r\n]A does match that string, because it con-
! 2105: tains an explicit CR or LF reference, and so advances only by one char-
! 2106: acter after the first failure.
! 2107:
! 2108: An explicit match for CR of LF is either a literal appearance of one of
! 2109: those characters, or one of the \r or \n escape sequences. Implicit
! 2110: matches such as [^X] do not count, nor does \s (which includes CR and
! 2111: LF in the characters that it matches).
! 2112:
! 2113: Notwithstanding the above, anomalous effects may still occur when CRLF
! 2114: is a valid newline sequence and explicit \r or \n escapes appear in the
! 2115: pattern.
! 2116:
! 2117: PCRE_NOTBOL
! 2118:
! 2119: This option specifies that first character of the subject string is not
! 2120: the beginning of a line, so the circumflex metacharacter should not
! 2121: match before it. Setting this without PCRE_MULTILINE (at compile time)
! 2122: causes circumflex never to match. This option affects only the behav-
! 2123: iour of the circumflex metacharacter. It does not affect \A.
! 2124:
! 2125: PCRE_NOTEOL
! 2126:
! 2127: This option specifies that the end of the subject string is not the end
! 2128: of a line, so the dollar metacharacter should not match it nor (except
! 2129: in multiline mode) a newline immediately before it. Setting this with-
! 2130: out PCRE_MULTILINE (at compile time) causes dollar never to match. This
! 2131: option affects only the behaviour of the dollar metacharacter. It does
! 2132: not affect \Z or \z.
! 2133:
! 2134: PCRE_NOTEMPTY
! 2135:
! 2136: An empty string is not considered to be a valid match if this option is
! 2137: set. If there are alternatives in the pattern, they are tried. If all
! 2138: the alternatives match the empty string, the entire match fails. For
! 2139: example, if the pattern
! 2140:
! 2141: a?b?
! 2142:
! 2143: is applied to a string not beginning with "a" or "b", it matches an
! 2144: empty string at the start of the subject. With PCRE_NOTEMPTY set, this
! 2145: match is not valid, so PCRE searches further into the string for occur-
! 2146: rences of "a" or "b".
! 2147:
! 2148: PCRE_NOTEMPTY_ATSTART
! 2149:
! 2150: This is like PCRE_NOTEMPTY, except that an empty string match that is
! 2151: not at the start of the subject is permitted. If the pattern is
! 2152: anchored, such a match can occur only if the pattern contains \K.
! 2153:
! 2154: Perl has no direct equivalent of PCRE_NOTEMPTY or
! 2155: PCRE_NOTEMPTY_ATSTART, but it does make a special case of a pattern
! 2156: match of the empty string within its split() function, and when using
! 2157: the /g modifier. It is possible to emulate Perl's behaviour after
! 2158: matching a null string by first trying the match again at the same off-
! 2159: set with PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED, and then if that
! 2160: fails, by advancing the starting offset (see below) and trying an ordi-
! 2161: nary match again. There is some code that demonstrates how to do this
! 2162: in the pcredemo sample program. In the most general case, you have to
! 2163: check to see if the newline convention recognizes CRLF as a newline,
! 2164: and if so, and the current character is CR followed by LF, advance the
! 2165: starting offset by two characters instead of one.
! 2166:
! 2167: PCRE_NO_START_OPTIMIZE
! 2168:
! 2169: There are a number of optimizations that pcre_exec() uses at the start
! 2170: of a match, in order to speed up the process. For example, if it is
! 2171: known that an unanchored match must start with a specific character, it
! 2172: searches the subject for that character, and fails immediately if it
! 2173: cannot find it, without actually running the main matching function.
! 2174: This means that a special item such as (*COMMIT) at the start of a pat-
! 2175: tern is not considered until after a suitable starting point for the
! 2176: match has been found. When callouts or (*MARK) items are in use, these
! 2177: "start-up" optimizations can cause them to be skipped if the pattern is
! 2178: never actually used. The start-up optimizations are in effect a pre-
! 2179: scan of the subject that takes place before the pattern is run.
! 2180:
! 2181: The PCRE_NO_START_OPTIMIZE option disables the start-up optimizations,
! 2182: possibly causing performance to suffer, but ensuring that in cases
! 2183: where the result is "no match", the callouts do occur, and that items
! 2184: such as (*COMMIT) and (*MARK) are considered at every possible starting
! 2185: position in the subject string. If PCRE_NO_START_OPTIMIZE is set at
! 2186: compile time, it cannot be unset at matching time.
! 2187:
! 2188: Setting PCRE_NO_START_OPTIMIZE can change the outcome of a matching
! 2189: operation. Consider the pattern
! 2190:
! 2191: (*COMMIT)ABC
! 2192:
! 2193: When this is compiled, PCRE records the fact that a match must start
! 2194: with the character "A". Suppose the subject string is "DEFABC". The
! 2195: start-up optimization scans along the subject, finds "A" and runs the
! 2196: first match attempt from there. The (*COMMIT) item means that the pat-
! 2197: tern must match the current starting position, which in this case, it
! 2198: does. However, if the same match is run with PCRE_NO_START_OPTIMIZE
! 2199: set, the initial scan along the subject string does not happen. The
! 2200: first match attempt is run starting from "D" and when this fails,
! 2201: (*COMMIT) prevents any further matches being tried, so the overall
! 2202: result is "no match". If the pattern is studied, more start-up opti-
! 2203: mizations may be used. For example, a minimum length for the subject
! 2204: may be recorded. Consider the pattern
! 2205:
! 2206: (*MARK:A)(X|Y)
! 2207:
! 2208: The minimum length for a match is one character. If the subject is
! 2209: "ABC", there will be attempts to match "ABC", "BC", "C", and then
! 2210: finally an empty string. If the pattern is studied, the final attempt
! 2211: does not take place, because PCRE knows that the subject is too short,
! 2212: and so the (*MARK) is never encountered. In this case, studying the
! 2213: pattern does not affect the overall match result, which is still "no
! 2214: match", but it does affect the auxiliary information that is returned.
! 2215:
! 2216: PCRE_NO_UTF8_CHECK
! 2217:
! 2218: When PCRE_UTF8 is set at compile time, the validity of the subject as a
! 2219: UTF-8 string is automatically checked when pcre_exec() is subsequently
! 2220: called. The value of startoffset is also checked to ensure that it
! 2221: points to the start of a UTF-8 character. There is a discussion about
! 2222: the validity of UTF-8 strings in the section on UTF-8 support in the
! 2223: main pcre page. If an invalid UTF-8 sequence of bytes is found,
! 2224: pcre_exec() returns the error PCRE_ERROR_BADUTF8 or, if PCRE_PAR-
! 2225: TIAL_HARD is set and the problem is a truncated UTF-8 character at the
! 2226: end of the subject, PCRE_ERROR_SHORTUTF8. In both cases, information
! 2227: about the precise nature of the error may also be returned (see the
! 2228: descriptions of these errors in the section entitled Error return val-
! 2229: ues from pcre_exec() below). If startoffset contains a value that does
! 2230: not point to the start of a UTF-8 character (or to the end of the sub-
! 2231: ject), PCRE_ERROR_BADUTF8_OFFSET is returned.
! 2232:
! 2233: If you already know that your subject is valid, and you want to skip
! 2234: these checks for performance reasons, you can set the
! 2235: PCRE_NO_UTF8_CHECK option when calling pcre_exec(). You might want to
! 2236: do this for the second and subsequent calls to pcre_exec() if you are
! 2237: making repeated calls to find all the matches in a single subject
! 2238: string. However, you should be sure that the value of startoffset
! 2239: points to the start of a UTF-8 character (or the end of the subject).
! 2240: When PCRE_NO_UTF8_CHECK is set, the effect of passing an invalid UTF-8
! 2241: string as a subject or an invalid value of startoffset is undefined.
! 2242: Your program may crash.
! 2243:
! 2244: PCRE_PARTIAL_HARD
! 2245: PCRE_PARTIAL_SOFT
! 2246:
! 2247: These options turn on the partial matching feature. For backwards com-
! 2248: patibility, PCRE_PARTIAL is a synonym for PCRE_PARTIAL_SOFT. A partial
! 2249: match occurs if the end of the subject string is reached successfully,
! 2250: but there are not enough subject characters to complete the match. If
! 2251: this happens when PCRE_PARTIAL_SOFT (but not PCRE_PARTIAL_HARD) is set,
! 2252: matching continues by testing any remaining alternatives. Only if no
! 2253: complete match can be found is PCRE_ERROR_PARTIAL returned instead of
! 2254: PCRE_ERROR_NOMATCH. In other words, PCRE_PARTIAL_SOFT says that the
! 2255: caller is prepared to handle a partial match, but only if no complete
! 2256: match can be found.
! 2257:
! 2258: If PCRE_PARTIAL_HARD is set, it overrides PCRE_PARTIAL_SOFT. In this
! 2259: case, if a partial match is found, pcre_exec() immediately returns
! 2260: PCRE_ERROR_PARTIAL, without considering any other alternatives. In
! 2261: other words, when PCRE_PARTIAL_HARD is set, a partial match is consid-
! 2262: ered to be more important that an alternative complete match.
! 2263:
! 2264: In both cases, the portion of the string that was inspected when the
! 2265: partial match was found is set as the first matching string. There is a
! 2266: more detailed discussion of partial and multi-segment matching, with
! 2267: examples, in the pcrepartial documentation.
! 2268:
! 2269: The string to be matched by pcre_exec()
! 2270:
! 2271: The subject string is passed to pcre_exec() as a pointer in subject, a
! 2272: length (in bytes) in length, and a starting byte offset in startoffset.
! 2273: If this is negative or greater than the length of the subject,
! 2274: pcre_exec() returns PCRE_ERROR_BADOFFSET. When the starting offset is
! 2275: zero, the search for a match starts at the beginning of the subject,
! 2276: and this is by far the most common case. In UTF-8 mode, the byte offset
! 2277: must point to the start of a UTF-8 character (or the end of the sub-
! 2278: ject). Unlike the pattern string, the subject may contain binary zero
! 2279: bytes.
! 2280:
! 2281: A non-zero starting offset is useful when searching for another match
! 2282: in the same subject by calling pcre_exec() again after a previous suc-
! 2283: cess. Setting startoffset differs from just passing over a shortened
! 2284: string and setting PCRE_NOTBOL in the case of a pattern that begins
! 2285: with any kind of lookbehind. For example, consider the pattern
! 2286:
! 2287: \Biss\B
! 2288:
! 2289: which finds occurrences of "iss" in the middle of words. (\B matches
! 2290: only if the current position in the subject is not a word boundary.)
! 2291: When applied to the string "Mississipi" the first call to pcre_exec()
! 2292: finds the first occurrence. If pcre_exec() is called again with just
! 2293: the remainder of the subject, namely "issipi", it does not match,
! 2294: because \B is always false at the start of the subject, which is deemed
! 2295: to be a word boundary. However, if pcre_exec() is passed the entire
! 2296: string again, but with startoffset set to 4, it finds the second occur-
! 2297: rence of "iss" because it is able to look behind the starting point to
! 2298: discover that it is preceded by a letter.
! 2299:
! 2300: Finding all the matches in a subject is tricky when the pattern can
! 2301: match an empty string. It is possible to emulate Perl's /g behaviour by
! 2302: first trying the match again at the same offset, with the
! 2303: PCRE_NOTEMPTY_ATSTART and PCRE_ANCHORED options, and then if that
! 2304: fails, advancing the starting offset and trying an ordinary match
! 2305: again. There is some code that demonstrates how to do this in the pcre-
! 2306: demo sample program. In the most general case, you have to check to see
! 2307: if the newline convention recognizes CRLF as a newline, and if so, and
! 2308: the current character is CR followed by LF, advance the starting offset
! 2309: by two characters instead of one.
! 2310:
! 2311: If a non-zero starting offset is passed when the pattern is anchored,
! 2312: one attempt to match at the given offset is made. This can only succeed
! 2313: if the pattern does not require the match to be at the start of the
! 2314: subject.
! 2315:
! 2316: How pcre_exec() returns captured substrings
! 2317:
! 2318: In general, a pattern matches a certain portion of the subject, and in
! 2319: addition, further substrings from the subject may be picked out by
! 2320: parts of the pattern. Following the usage in Jeffrey Friedl's book,
! 2321: this is called "capturing" in what follows, and the phrase "capturing
! 2322: subpattern" is used for a fragment of a pattern that picks out a sub-
! 2323: string. PCRE supports several other kinds of parenthesized subpattern
! 2324: that do not cause substrings to be captured.
! 2325:
! 2326: Captured substrings are returned to the caller via a vector of integers
! 2327: whose address is passed in ovector. The number of elements in the vec-
! 2328: tor is passed in ovecsize, which must be a non-negative number. Note:
! 2329: this argument is NOT the size of ovector in bytes.
! 2330:
! 2331: The first two-thirds of the vector is used to pass back captured sub-
! 2332: strings, each substring using a pair of integers. The remaining third
! 2333: of the vector is used as workspace by pcre_exec() while matching cap-
! 2334: turing subpatterns, and is not available for passing back information.
! 2335: The number passed in ovecsize should always be a multiple of three. If
! 2336: it is not, it is rounded down.
! 2337:
! 2338: When a match is successful, information about captured substrings is
! 2339: returned in pairs of integers, starting at the beginning of ovector,
! 2340: and continuing up to two-thirds of its length at the most. The first
! 2341: element of each pair is set to the byte offset of the first character
! 2342: in a substring, and the second is set to the byte offset of the first
! 2343: character after the end of a substring. Note: these values are always
! 2344: byte offsets, even in UTF-8 mode. They are not character counts.
! 2345:
! 2346: The first pair of integers, ovector[0] and ovector[1], identify the
! 2347: portion of the subject string matched by the entire pattern. The next
! 2348: pair is used for the first capturing subpattern, and so on. The value
! 2349: returned by pcre_exec() is one more than the highest numbered pair that
! 2350: has been set. For example, if two substrings have been captured, the
! 2351: returned value is 3. If there are no capturing subpatterns, the return
! 2352: value from a successful match is 1, indicating that just the first pair
! 2353: of offsets has been set.
! 2354:
! 2355: If a capturing subpattern is matched repeatedly, it is the last portion
! 2356: of the string that it matched that is returned.
! 2357:
! 2358: If the vector is too small to hold all the captured substring offsets,
! 2359: it is used as far as possible (up to two-thirds of its length), and the
! 2360: function returns a value of zero. If neither the actual string matched
! 2361: not any captured substrings are of interest, pcre_exec() may be called
! 2362: with ovector passed as NULL and ovecsize as zero. However, if the pat-
! 2363: tern contains back references and the ovector is not big enough to
! 2364: remember the related substrings, PCRE has to get additional memory for
! 2365: use during matching. Thus it is usually advisable to supply an ovector
! 2366: of reasonable size.
! 2367:
! 2368: There are some cases where zero is returned (indicating vector over-
! 2369: flow) when in fact the vector is exactly the right size for the final
! 2370: match. For example, consider the pattern
! 2371:
! 2372: (a)(?:(b)c|bd)
! 2373:
! 2374: If a vector of 6 elements (allowing for only 1 captured substring) is
! 2375: given with subject string "abd", pcre_exec() will try to set the second
! 2376: captured string, thereby recording a vector overflow, before failing to
! 2377: match "c" and backing up to try the second alternative. The zero
! 2378: return, however, does correctly indicate that the maximum number of
! 2379: slots (namely 2) have been filled. In similar cases where there is tem-
! 2380: porary overflow, but the final number of used slots is actually less
! 2381: than the maximum, a non-zero value is returned.
! 2382:
! 2383: The pcre_fullinfo() function can be used to find out how many capturing
! 2384: subpatterns there are in a compiled pattern. The smallest size for
! 2385: ovector that will allow for n captured substrings, in addition to the
! 2386: offsets of the substring matched by the whole pattern, is (n+1)*3.
! 2387:
! 2388: It is possible for capturing subpattern number n+1 to match some part
! 2389: of the subject when subpattern n has not been used at all. For example,
! 2390: if the string "abc" is matched against the pattern (a|(z))(bc) the
! 2391: return from the function is 4, and subpatterns 1 and 3 are matched, but
! 2392: 2 is not. When this happens, both values in the offset pairs corre-
! 2393: sponding to unused subpatterns are set to -1.
! 2394:
! 2395: Offset values that correspond to unused subpatterns at the end of the
! 2396: expression are also set to -1. For example, if the string "abc" is
! 2397: matched against the pattern (abc)(x(yz)?)? subpatterns 2 and 3 are not
! 2398: matched. The return from the function is 2, because the highest used
! 2399: capturing subpattern number is 1, and the offsets for for the second
! 2400: and third capturing subpatterns (assuming the vector is large enough,
! 2401: of course) are set to -1.
! 2402:
! 2403: Note: Elements in the first two-thirds of ovector that do not corre-
! 2404: spond to capturing parentheses in the pattern are never changed. That
! 2405: is, if a pattern contains n capturing parentheses, no more than ovec-
! 2406: tor[0] to ovector[2n+1] are set by pcre_exec(). The other elements (in
! 2407: the first two-thirds) retain whatever values they previously had.
! 2408:
! 2409: Some convenience functions are provided for extracting the captured
! 2410: substrings as separate strings. These are described below.
! 2411:
! 2412: Error return values from pcre_exec()
! 2413:
! 2414: If pcre_exec() fails, it returns a negative number. The following are
! 2415: defined in the header file:
! 2416:
! 2417: PCRE_ERROR_NOMATCH (-1)
! 2418:
! 2419: The subject string did not match the pattern.
! 2420:
! 2421: PCRE_ERROR_NULL (-2)
! 2422:
! 2423: Either code or subject was passed as NULL, or ovector was NULL and
! 2424: ovecsize was not zero.
! 2425:
! 2426: PCRE_ERROR_BADOPTION (-3)
! 2427:
! 2428: An unrecognized bit was set in the options argument.
! 2429:
! 2430: PCRE_ERROR_BADMAGIC (-4)
! 2431:
! 2432: PCRE stores a 4-byte "magic number" at the start of the compiled code,
! 2433: to catch the case when it is passed a junk pointer and to detect when a
! 2434: pattern that was compiled in an environment of one endianness is run in
! 2435: an environment with the other endianness. This is the error that PCRE
! 2436: gives when the magic number is not present.
! 2437:
! 2438: PCRE_ERROR_UNKNOWN_OPCODE (-5)
! 2439:
! 2440: While running the pattern match, an unknown item was encountered in the
! 2441: compiled pattern. This error could be caused by a bug in PCRE or by
! 2442: overwriting of the compiled pattern.
! 2443:
! 2444: PCRE_ERROR_NOMEMORY (-6)
! 2445:
! 2446: If a pattern contains back references, but the ovector that is passed
! 2447: to pcre_exec() is not big enough to remember the referenced substrings,
! 2448: PCRE gets a block of memory at the start of matching to use for this
! 2449: purpose. If the call via pcre_malloc() fails, this error is given. The
! 2450: memory is automatically freed at the end of matching.
! 2451:
! 2452: This error is also given if pcre_stack_malloc() fails in pcre_exec().
! 2453: This can happen only when PCRE has been compiled with --disable-stack-
! 2454: for-recursion.
! 2455:
! 2456: PCRE_ERROR_NOSUBSTRING (-7)
! 2457:
! 2458: This error is used by the pcre_copy_substring(), pcre_get_substring(),
! 2459: and pcre_get_substring_list() functions (see below). It is never
! 2460: returned by pcre_exec().
! 2461:
! 2462: PCRE_ERROR_MATCHLIMIT (-8)
! 2463:
! 2464: The backtracking limit, as specified by the match_limit field in a
! 2465: pcre_extra structure (or defaulted) was reached. See the description
! 2466: above.
! 2467:
! 2468: PCRE_ERROR_CALLOUT (-9)
! 2469:
! 2470: This error is never generated by pcre_exec() itself. It is provided for
! 2471: use by callout functions that want to yield a distinctive error code.
! 2472: See the pcrecallout documentation for details.
! 2473:
! 2474: PCRE_ERROR_BADUTF8 (-10)
! 2475:
! 2476: A string that contains an invalid UTF-8 byte sequence was passed as a
! 2477: subject, and the PCRE_NO_UTF8_CHECK option was not set. If the size of
! 2478: the output vector (ovecsize) is at least 2, the byte offset to the
! 2479: start of the the invalid UTF-8 character is placed in the first ele-
! 2480: ment, and a reason code is placed in the second element. The reason
! 2481: codes are listed in the following section. For backward compatibility,
! 2482: if PCRE_PARTIAL_HARD is set and the problem is a truncated UTF-8 char-
! 2483: acter at the end of the subject (reason codes 1 to 5),
! 2484: PCRE_ERROR_SHORTUTF8 is returned instead of PCRE_ERROR_BADUTF8.
! 2485:
! 2486: PCRE_ERROR_BADUTF8_OFFSET (-11)
! 2487:
! 2488: The UTF-8 byte sequence that was passed as a subject was checked and
! 2489: found to be valid (the PCRE_NO_UTF8_CHECK option was not set), but the
! 2490: value of startoffset did not point to the beginning of a UTF-8 charac-
! 2491: ter or the end of the subject.
! 2492:
! 2493: PCRE_ERROR_PARTIAL (-12)
! 2494:
! 2495: The subject string did not match, but it did match partially. See the
! 2496: pcrepartial documentation for details of partial matching.
! 2497:
! 2498: PCRE_ERROR_BADPARTIAL (-13)
! 2499:
! 2500: This code is no longer in use. It was formerly returned when the
! 2501: PCRE_PARTIAL option was used with a compiled pattern containing items
! 2502: that were not supported for partial matching. From release 8.00
! 2503: onwards, there are no restrictions on partial matching.
! 2504:
! 2505: PCRE_ERROR_INTERNAL (-14)
! 2506:
! 2507: An unexpected internal error has occurred. This error could be caused
! 2508: by a bug in PCRE or by overwriting of the compiled pattern.
! 2509:
! 2510: PCRE_ERROR_BADCOUNT (-15)
! 2511:
! 2512: This error is given if the value of the ovecsize argument is negative.
! 2513:
! 2514: PCRE_ERROR_RECURSIONLIMIT (-21)
! 2515:
! 2516: The internal recursion limit, as specified by the match_limit_recursion
! 2517: field in a pcre_extra structure (or defaulted) was reached. See the
! 2518: description above.
! 2519:
! 2520: PCRE_ERROR_BADNEWLINE (-23)
! 2521:
! 2522: An invalid combination of PCRE_NEWLINE_xxx options was given.
! 2523:
! 2524: PCRE_ERROR_BADOFFSET (-24)
! 2525:
! 2526: The value of startoffset was negative or greater than the length of the
! 2527: subject, that is, the value in length.
! 2528:
! 2529: PCRE_ERROR_SHORTUTF8 (-25)
! 2530:
! 2531: This error is returned instead of PCRE_ERROR_BADUTF8 when the subject
! 2532: string ends with a truncated UTF-8 character and the PCRE_PARTIAL_HARD
! 2533: option is set. Information about the failure is returned as for
! 2534: PCRE_ERROR_BADUTF8. It is in fact sufficient to detect this case, but
! 2535: this special error code for PCRE_PARTIAL_HARD precedes the implementa-
! 2536: tion of returned information; it is retained for backwards compatibil-
! 2537: ity.
! 2538:
! 2539: PCRE_ERROR_RECURSELOOP (-26)
! 2540:
! 2541: This error is returned when pcre_exec() detects a recursion loop within
! 2542: the pattern. Specifically, it means that either the whole pattern or a
! 2543: subpattern has been called recursively for the second time at the same
! 2544: position in the subject string. Some simple patterns that might do this
! 2545: are detected and faulted at compile time, but more complicated cases,
! 2546: in particular mutual recursions between two different subpatterns, can-
! 2547: not be detected until run time.
! 2548:
! 2549: PCRE_ERROR_JIT_STACKLIMIT (-27)
! 2550:
! 2551: This error is returned when a pattern that was successfully studied
! 2552: using the PCRE_STUDY_JIT_COMPILE option is being matched, but the mem-
! 2553: ory available for the just-in-time processing stack is not large
! 2554: enough. See the pcrejit documentation for more details.
! 2555:
! 2556: Error numbers -16 to -20 and -22 are not used by pcre_exec().
! 2557:
! 2558: Reason codes for invalid UTF-8 strings
! 2559:
! 2560: When pcre_exec() returns either PCRE_ERROR_BADUTF8 or PCRE_ERROR_SHORT-
! 2561: UTF8, and the size of the output vector (ovecsize) is at least 2, the
! 2562: offset of the start of the invalid UTF-8 character is placed in the
! 2563: first output vector element (ovector[0]) and a reason code is placed in
! 2564: the second element (ovector[1]). The reason codes are given names in
! 2565: the pcre.h header file:
! 2566:
! 2567: PCRE_UTF8_ERR1
! 2568: PCRE_UTF8_ERR2
! 2569: PCRE_UTF8_ERR3
! 2570: PCRE_UTF8_ERR4
! 2571: PCRE_UTF8_ERR5
! 2572:
! 2573: The string ends with a truncated UTF-8 character; the code specifies
! 2574: how many bytes are missing (1 to 5). Although RFC 3629 restricts UTF-8
! 2575: characters to be no longer than 4 bytes, the encoding scheme (origi-
! 2576: nally defined by RFC 2279) allows for up to 6 bytes, and this is
! 2577: checked first; hence the possibility of 4 or 5 missing bytes.
! 2578:
! 2579: PCRE_UTF8_ERR6
! 2580: PCRE_UTF8_ERR7
! 2581: PCRE_UTF8_ERR8
! 2582: PCRE_UTF8_ERR9
! 2583: PCRE_UTF8_ERR10
! 2584:
! 2585: The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
! 2586: the character do not have the binary value 0b10 (that is, either the
! 2587: most significant bit is 0, or the next bit is 1).
! 2588:
! 2589: PCRE_UTF8_ERR11
! 2590: PCRE_UTF8_ERR12
! 2591:
! 2592: A character that is valid by the RFC 2279 rules is either 5 or 6 bytes
! 2593: long; these code points are excluded by RFC 3629.
! 2594:
! 2595: PCRE_UTF8_ERR13
! 2596:
! 2597: A 4-byte character has a value greater than 0x10fff; these code points
! 2598: are excluded by RFC 3629.
! 2599:
! 2600: PCRE_UTF8_ERR14
! 2601:
! 2602: A 3-byte character has a value in the range 0xd800 to 0xdfff; this
! 2603: range of code points are reserved by RFC 3629 for use with UTF-16, and
! 2604: so are excluded from UTF-8.
! 2605:
! 2606: PCRE_UTF8_ERR15
! 2607: PCRE_UTF8_ERR16
! 2608: PCRE_UTF8_ERR17
! 2609: PCRE_UTF8_ERR18
! 2610: PCRE_UTF8_ERR19
! 2611:
! 2612: A 2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes
! 2613: for a value that can be represented by fewer bytes, which is invalid.
! 2614: For example, the two bytes 0xc0, 0xae give the value 0x2e, whose cor-
! 2615: rect coding uses just one byte.
! 2616:
! 2617: PCRE_UTF8_ERR20
! 2618:
! 2619: The two most significant bits of the first byte of a character have the
! 2620: binary value 0b10 (that is, the most significant bit is 1 and the sec-
! 2621: ond is 0). Such a byte can only validly occur as the second or subse-
! 2622: quent byte of a multi-byte character.
! 2623:
! 2624: PCRE_UTF8_ERR21
! 2625:
! 2626: The first byte of a character has the value 0xfe or 0xff. These values
! 2627: can never occur in a valid UTF-8 string.
! 2628:
! 2629:
! 2630: EXTRACTING CAPTURED SUBSTRINGS BY NUMBER
! 2631:
! 2632: int pcre_copy_substring(const char *subject, int *ovector,
! 2633: int stringcount, int stringnumber, char *buffer,
! 2634: int buffersize);
! 2635:
! 2636: int pcre_get_substring(const char *subject, int *ovector,
! 2637: int stringcount, int stringnumber,
! 2638: const char **stringptr);
! 2639:
! 2640: int pcre_get_substring_list(const char *subject,
! 2641: int *ovector, int stringcount, const char ***listptr);
! 2642:
! 2643: Captured substrings can be accessed directly by using the offsets
! 2644: returned by pcre_exec() in ovector. For convenience, the functions
! 2645: pcre_copy_substring(), pcre_get_substring(), and pcre_get_sub-
! 2646: string_list() are provided for extracting captured substrings as new,
! 2647: separate, zero-terminated strings. These functions identify substrings
! 2648: by number. The next section describes functions for extracting named
! 2649: substrings.
! 2650:
! 2651: A substring that contains a binary zero is correctly extracted and has
! 2652: a further zero added on the end, but the result is not, of course, a C
! 2653: string. However, you can process such a string by referring to the
! 2654: length that is returned by pcre_copy_substring() and pcre_get_sub-
! 2655: string(). Unfortunately, the interface to pcre_get_substring_list() is
! 2656: not adequate for handling strings containing binary zeros, because the
! 2657: end of the final string is not independently indicated.
! 2658:
! 2659: The first three arguments are the same for all three of these func-
! 2660: tions: subject is the subject string that has just been successfully
! 2661: matched, ovector is a pointer to the vector of integer offsets that was
! 2662: passed to pcre_exec(), and stringcount is the number of substrings that
! 2663: were captured by the match, including the substring that matched the
! 2664: entire regular expression. This is the value returned by pcre_exec() if
! 2665: it is greater than zero. If pcre_exec() returned zero, indicating that
! 2666: it ran out of space in ovector, the value passed as stringcount should
! 2667: be the number of elements in the vector divided by three.
! 2668:
! 2669: The functions pcre_copy_substring() and pcre_get_substring() extract a
! 2670: single substring, whose number is given as stringnumber. A value of
! 2671: zero extracts the substring that matched the entire pattern, whereas
! 2672: higher values extract the captured substrings. For pcre_copy_sub-
! 2673: string(), the string is placed in buffer, whose length is given by
! 2674: buffersize, while for pcre_get_substring() a new block of memory is
! 2675: obtained via pcre_malloc, and its address is returned via stringptr.
! 2676: The yield of the function is the length of the string, not including
! 2677: the terminating zero, or one of these error codes:
! 2678:
! 2679: PCRE_ERROR_NOMEMORY (-6)
! 2680:
! 2681: The buffer was too small for pcre_copy_substring(), or the attempt to
! 2682: get memory failed for pcre_get_substring().
! 2683:
! 2684: PCRE_ERROR_NOSUBSTRING (-7)
! 2685:
! 2686: There is no substring whose number is stringnumber.
! 2687:
! 2688: The pcre_get_substring_list() function extracts all available sub-
! 2689: strings and builds a list of pointers to them. All this is done in a
! 2690: single block of memory that is obtained via pcre_malloc. The address of
! 2691: the memory block is returned via listptr, which is also the start of
! 2692: the list of string pointers. The end of the list is marked by a NULL
! 2693: pointer. The yield of the function is zero if all went well, or the
! 2694: error code
! 2695:
! 2696: PCRE_ERROR_NOMEMORY (-6)
! 2697:
! 2698: if the attempt to get the memory block failed.
! 2699:
! 2700: When any of these functions encounter a substring that is unset, which
! 2701: can happen when capturing subpattern number n+1 matches some part of
! 2702: the subject, but subpattern n has not been used at all, they return an
! 2703: empty string. This can be distinguished from a genuine zero-length sub-
! 2704: string by inspecting the appropriate offset in ovector, which is nega-
! 2705: tive for unset substrings.
! 2706:
! 2707: The two convenience functions pcre_free_substring() and pcre_free_sub-
! 2708: string_list() can be used to free the memory returned by a previous
! 2709: call of pcre_get_substring() or pcre_get_substring_list(), respec-
! 2710: tively. They do nothing more than call the function pointed to by
! 2711: pcre_free, which of course could be called directly from a C program.
! 2712: However, PCRE is used in some situations where it is linked via a spe-
! 2713: cial interface to another programming language that cannot use
! 2714: pcre_free directly; it is for these cases that the functions are pro-
! 2715: vided.
! 2716:
! 2717:
! 2718: EXTRACTING CAPTURED SUBSTRINGS BY NAME
! 2719:
! 2720: int pcre_get_stringnumber(const pcre *code,
! 2721: const char *name);
! 2722:
! 2723: int pcre_copy_named_substring(const pcre *code,
! 2724: const char *subject, int *ovector,
! 2725: int stringcount, const char *stringname,
! 2726: char *buffer, int buffersize);
! 2727:
! 2728: int pcre_get_named_substring(const pcre *code,
! 2729: const char *subject, int *ovector,
! 2730: int stringcount, const char *stringname,
! 2731: const char **stringptr);
! 2732:
! 2733: To extract a substring by name, you first have to find associated num-
! 2734: ber. For example, for this pattern
! 2735:
! 2736: (a+)b(?<xxx>\d+)...
! 2737:
! 2738: the number of the subpattern called "xxx" is 2. If the name is known to
! 2739: be unique (PCRE_DUPNAMES was not set), you can find the number from the
! 2740: name by calling pcre_get_stringnumber(). The first argument is the com-
! 2741: piled pattern, and the second is the name. The yield of the function is
! 2742: the subpattern number, or PCRE_ERROR_NOSUBSTRING (-7) if there is no
! 2743: subpattern of that name.
! 2744:
! 2745: Given the number, you can extract the substring directly, or use one of
! 2746: the functions described in the previous section. For convenience, there
! 2747: are also two functions that do the whole job.
! 2748:
! 2749: Most of the arguments of pcre_copy_named_substring() and
! 2750: pcre_get_named_substring() are the same as those for the similarly
! 2751: named functions that extract by number. As these are described in the
! 2752: previous section, they are not re-described here. There are just two
! 2753: differences:
! 2754:
! 2755: First, instead of a substring number, a substring name is given. Sec-
! 2756: ond, there is an extra argument, given at the start, which is a pointer
! 2757: to the compiled pattern. This is needed in order to gain access to the
! 2758: name-to-number translation table.
! 2759:
! 2760: These functions call pcre_get_stringnumber(), and if it succeeds, they
! 2761: then call pcre_copy_substring() or pcre_get_substring(), as appropri-
! 2762: ate. NOTE: If PCRE_DUPNAMES is set and there are duplicate names, the
! 2763: behaviour may not be what you want (see the next section).
! 2764:
! 2765: Warning: If the pattern uses the (?| feature to set up multiple subpat-
! 2766: terns with the same number, as described in the section on duplicate
! 2767: subpattern numbers in the pcrepattern page, you cannot use names to
! 2768: distinguish the different subpatterns, because names are not included
! 2769: in the compiled code. The matching process uses only numbers. For this
! 2770: reason, the use of different names for subpatterns of the same number
! 2771: causes an error at compile time.
! 2772:
! 2773:
! 2774: DUPLICATE SUBPATTERN NAMES
! 2775:
! 2776: int pcre_get_stringtable_entries(const pcre *code,
! 2777: const char *name, char **first, char **last);
! 2778:
! 2779: When a pattern is compiled with the PCRE_DUPNAMES option, names for
! 2780: subpatterns are not required to be unique. (Duplicate names are always
! 2781: allowed for subpatterns with the same number, created by using the (?|
! 2782: feature. Indeed, if such subpatterns are named, they are required to
! 2783: use the same names.)
! 2784:
! 2785: Normally, patterns with duplicate names are such that in any one match,
! 2786: only one of the named subpatterns participates. An example is shown in
! 2787: the pcrepattern documentation.
! 2788:
! 2789: When duplicates are present, pcre_copy_named_substring() and
! 2790: pcre_get_named_substring() return the first substring corresponding to
! 2791: the given name that is set. If none are set, PCRE_ERROR_NOSUBSTRING
! 2792: (-7) is returned; no data is returned. The pcre_get_stringnumber()
! 2793: function returns one of the numbers that are associated with the name,
! 2794: but it is not defined which it is.
! 2795:
! 2796: If you want to get full details of all captured substrings for a given
! 2797: name, you must use the pcre_get_stringtable_entries() function. The
! 2798: first argument is the compiled pattern, and the second is the name. The
! 2799: third and fourth are pointers to variables which are updated by the
! 2800: function. After it has run, they point to the first and last entries in
! 2801: the name-to-number table for the given name. The function itself
! 2802: returns the length of each entry, or PCRE_ERROR_NOSUBSTRING (-7) if
! 2803: there are none. The format of the table is described above in the sec-
! 2804: tion entitled Information about a pattern above. Given all the rele-
! 2805: vant entries for the name, you can extract each of their numbers, and
! 2806: hence the captured data, if any.
! 2807:
! 2808:
! 2809: FINDING ALL POSSIBLE MATCHES
! 2810:
! 2811: The traditional matching function uses a similar algorithm to Perl,
! 2812: which stops when it finds the first match, starting at a given point in
! 2813: the subject. If you want to find all possible matches, or the longest
! 2814: possible match, consider using the alternative matching function (see
! 2815: below) instead. If you cannot use the alternative function, but still
! 2816: need to find all possible matches, you can kludge it up by making use
! 2817: of the callout facility, which is described in the pcrecallout documen-
! 2818: tation.
! 2819:
! 2820: What you have to do is to insert a callout right at the end of the pat-
! 2821: tern. When your callout function is called, extract and save the cur-
! 2822: rent matched substring. Then return 1, which forces pcre_exec() to
! 2823: backtrack and try other alternatives. Ultimately, when it runs out of
! 2824: matches, pcre_exec() will yield PCRE_ERROR_NOMATCH.
! 2825:
! 2826:
! 2827: MATCHING A PATTERN: THE ALTERNATIVE FUNCTION
! 2828:
! 2829: int pcre_dfa_exec(const pcre *code, const pcre_extra *extra,
! 2830: const char *subject, int length, int startoffset,
! 2831: int options, int *ovector, int ovecsize,
! 2832: int *workspace, int wscount);
! 2833:
! 2834: The function pcre_dfa_exec() is called to match a subject string
! 2835: against a compiled pattern, using a matching algorithm that scans the
! 2836: subject string just once, and does not backtrack. This has different
! 2837: characteristics to the normal algorithm, and is not compatible with
! 2838: Perl. Some of the features of PCRE patterns are not supported. Never-
! 2839: theless, there are times when this kind of matching can be useful. For
! 2840: a discussion of the two matching algorithms, and a list of features
! 2841: that pcre_dfa_exec() does not support, see the pcrematching documenta-
! 2842: tion.
! 2843:
! 2844: The arguments for the pcre_dfa_exec() function are the same as for
! 2845: pcre_exec(), plus two extras. The ovector argument is used in a differ-
! 2846: ent way, and this is described below. The other common arguments are
! 2847: used in the same way as for pcre_exec(), so their description is not
! 2848: repeated here.
! 2849:
! 2850: The two additional arguments provide workspace for the function. The
! 2851: workspace vector should contain at least 20 elements. It is used for
! 2852: keeping track of multiple paths through the pattern tree. More
! 2853: workspace will be needed for patterns and subjects where there are a
! 2854: lot of potential matches.
! 2855:
! 2856: Here is an example of a simple call to pcre_dfa_exec():
! 2857:
! 2858: int rc;
! 2859: int ovector[10];
! 2860: int wspace[20];
! 2861: rc = pcre_dfa_exec(
! 2862: re, /* result of pcre_compile() */
! 2863: NULL, /* we didn't study the pattern */
! 2864: "some string", /* the subject string */
! 2865: 11, /* the length of the subject string */
! 2866: 0, /* start at offset 0 in the subject */
! 2867: 0, /* default options */
! 2868: ovector, /* vector of integers for substring information */
! 2869: 10, /* number of elements (NOT size in bytes) */
! 2870: wspace, /* working space vector */
! 2871: 20); /* number of elements (NOT size in bytes) */
! 2872:
! 2873: Option bits for pcre_dfa_exec()
! 2874:
! 2875: The unused bits of the options argument for pcre_dfa_exec() must be
! 2876: zero. The only bits that may be set are PCRE_ANCHORED, PCRE_NEW-
! 2877: LINE_xxx, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY,
! 2878: PCRE_NOTEMPTY_ATSTART, PCRE_NO_UTF8_CHECK, PCRE_BSR_ANYCRLF,
! 2879: PCRE_BSR_UNICODE, PCRE_NO_START_OPTIMIZE, PCRE_PARTIAL_HARD, PCRE_PAR-
! 2880: TIAL_SOFT, PCRE_DFA_SHORTEST, and PCRE_DFA_RESTART. All but the last
! 2881: four of these are exactly the same as for pcre_exec(), so their
! 2882: description is not repeated here.
! 2883:
! 2884: PCRE_PARTIAL_HARD
! 2885: PCRE_PARTIAL_SOFT
! 2886:
! 2887: These have the same general effect as they do for pcre_exec(), but the
! 2888: details are slightly different. When PCRE_PARTIAL_HARD is set for
! 2889: pcre_dfa_exec(), it returns PCRE_ERROR_PARTIAL if the end of the sub-
! 2890: ject is reached and there is still at least one matching possibility
! 2891: that requires additional characters. This happens even if some complete
! 2892: matches have also been found. When PCRE_PARTIAL_SOFT is set, the return
! 2893: code PCRE_ERROR_NOMATCH is converted into PCRE_ERROR_PARTIAL if the end
! 2894: of the subject is reached, there have been no complete matches, but
! 2895: there is still at least one matching possibility. The portion of the
! 2896: string that was inspected when the longest partial match was found is
! 2897: set as the first matching string in both cases. There is a more
! 2898: detailed discussion of partial and multi-segment matching, with exam-
! 2899: ples, in the pcrepartial documentation.
! 2900:
! 2901: PCRE_DFA_SHORTEST
! 2902:
! 2903: Setting the PCRE_DFA_SHORTEST option causes the matching algorithm to
! 2904: stop as soon as it has found one match. Because of the way the alterna-
! 2905: tive algorithm works, this is necessarily the shortest possible match
! 2906: at the first possible matching point in the subject string.
! 2907:
! 2908: PCRE_DFA_RESTART
! 2909:
! 2910: When pcre_dfa_exec() returns a partial match, it is possible to call it
! 2911: again, with additional subject characters, and have it continue with
! 2912: the same match. The PCRE_DFA_RESTART option requests this action; when
! 2913: it is set, the workspace and wscount options must reference the same
! 2914: vector as before because data about the match so far is left in them
! 2915: after a partial match. There is more discussion of this facility in the
! 2916: pcrepartial documentation.
! 2917:
! 2918: Successful returns from pcre_dfa_exec()
! 2919:
! 2920: When pcre_dfa_exec() succeeds, it may have matched more than one sub-
! 2921: string in the subject. Note, however, that all the matches from one run
! 2922: of the function start at the same point in the subject. The shorter
! 2923: matches are all initial substrings of the longer matches. For example,
! 2924: if the pattern
! 2925:
! 2926: <.*>
! 2927:
! 2928: is matched against the string
! 2929:
! 2930: This is <something> <something else> <something further> no more
! 2931:
! 2932: the three matched strings are
! 2933:
! 2934: <something>
! 2935: <something> <something else>
! 2936: <something> <something else> <something further>
! 2937:
! 2938: On success, the yield of the function is a number greater than zero,
! 2939: which is the number of matched substrings. The substrings themselves
! 2940: are returned in ovector. Each string uses two elements; the first is
! 2941: the offset to the start, and the second is the offset to the end. In
! 2942: fact, all the strings have the same start offset. (Space could have
! 2943: been saved by giving this only once, but it was decided to retain some
! 2944: compatibility with the way pcre_exec() returns data, even though the
! 2945: meaning of the strings is different.)
! 2946:
! 2947: The strings are returned in reverse order of length; that is, the long-
! 2948: est matching string is given first. If there were too many matches to
! 2949: fit into ovector, the yield of the function is zero, and the vector is
! 2950: filled with the longest matches. Unlike pcre_exec(), pcre_dfa_exec()
! 2951: can use the entire ovector for returning matched strings.
! 2952:
! 2953: Error returns from pcre_dfa_exec()
! 2954:
! 2955: The pcre_dfa_exec() function returns a negative number when it fails.
! 2956: Many of the errors are the same as for pcre_exec(), and these are
! 2957: described above. There are in addition the following errors that are
! 2958: specific to pcre_dfa_exec():
! 2959:
! 2960: PCRE_ERROR_DFA_UITEM (-16)
! 2961:
! 2962: This return is given if pcre_dfa_exec() encounters an item in the pat-
! 2963: tern that it does not support, for instance, the use of \C or a back
! 2964: reference.
! 2965:
! 2966: PCRE_ERROR_DFA_UCOND (-17)
! 2967:
! 2968: This return is given if pcre_dfa_exec() encounters a condition item
! 2969: that uses a back reference for the condition, or a test for recursion
! 2970: in a specific group. These are not supported.
! 2971:
! 2972: PCRE_ERROR_DFA_UMLIMIT (-18)
! 2973:
! 2974: This return is given if pcre_dfa_exec() is called with an extra block
! 2975: that contains a setting of the match_limit or match_limit_recursion
! 2976: fields. This is not supported (these fields are meaningless for DFA
! 2977: matching).
! 2978:
! 2979: PCRE_ERROR_DFA_WSSIZE (-19)
! 2980:
! 2981: This return is given if pcre_dfa_exec() runs out of space in the
! 2982: workspace vector.
! 2983:
! 2984: PCRE_ERROR_DFA_RECURSE (-20)
! 2985:
! 2986: When a recursive subpattern is processed, the matching function calls
! 2987: itself recursively, using private vectors for ovector and workspace.
! 2988: This error is given if the output vector is not large enough. This
! 2989: should be extremely rare, as a vector of size 1000 is used.
! 2990:
! 2991:
! 2992: SEE ALSO
! 2993:
! 2994: pcrebuild(3), pcrecallout(3), pcrecpp(3)(3), pcrematching(3), pcrepar-
! 2995: tial(3), pcreposix(3), pcreprecompile(3), pcresample(3), pcrestack(3).
! 2996:
! 2997:
! 2998: AUTHOR
! 2999:
! 3000: Philip Hazel
! 3001: University Computing Service
! 3002: Cambridge CB2 3QH, England.
! 3003:
! 3004:
! 3005: REVISION
! 3006:
! 3007: Last updated: 02 December 2011
! 3008: Copyright (c) 1997-2011 University of Cambridge.
! 3009: ------------------------------------------------------------------------------
! 3010:
! 3011:
! 3012: PCRECALLOUT(3) PCRECALLOUT(3)
! 3013:
! 3014:
! 3015: NAME
! 3016: PCRE - Perl-compatible regular expressions
! 3017:
! 3018:
! 3019: PCRE CALLOUTS
! 3020:
! 3021: int (*pcre_callout)(pcre_callout_block *);
! 3022:
! 3023: PCRE provides a feature called "callout", which is a means of temporar-
! 3024: ily passing control to the caller of PCRE in the middle of pattern
! 3025: matching. The caller of PCRE provides an external function by putting
! 3026: its entry point in the global variable pcre_callout. By default, this
! 3027: variable contains NULL, which disables all calling out.
! 3028:
! 3029: Within a regular expression, (?C) indicates the points at which the
! 3030: external function is to be called. Different callout points can be
! 3031: identified by putting a number less than 256 after the letter C. The
! 3032: default value is zero. For example, this pattern has two callout
! 3033: points:
! 3034:
! 3035: (?C1)abc(?C2)def
! 3036:
! 3037: If the PCRE_AUTO_CALLOUT option bit is set when pcre_compile() or
! 3038: pcre_compile2() is called, PCRE automatically inserts callouts, all
! 3039: with number 255, before each item in the pattern. For example, if
! 3040: PCRE_AUTO_CALLOUT is used with the pattern
! 3041:
! 3042: A(\d{2}|--)
! 3043:
! 3044: it is processed as if it were
! 3045:
! 3046: (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)
! 3047:
! 3048: Notice that there is a callout before and after each parenthesis and
! 3049: alternation bar. Automatic callouts can be used for tracking the
! 3050: progress of pattern matching. The pcretest command has an option that
! 3051: sets automatic callouts; when it is used, the output indicates how the
! 3052: pattern is matched. This is useful information when you are trying to
! 3053: optimize the performance of a particular pattern.
! 3054:
! 3055: The use of callouts in a pattern makes it ineligible for optimization
! 3056: by the just-in-time compiler. Studying such a pattern with the
! 3057: PCRE_STUDY_JIT_COMPILE option always fails.
! 3058:
! 3059:
! 3060: MISSING CALLOUTS
! 3061:
! 3062: You should be aware that, because of optimizations in the way PCRE
! 3063: matches patterns by default, callouts sometimes do not happen. For
! 3064: example, if the pattern is
! 3065:
! 3066: ab(?C4)cd
! 3067:
! 3068: PCRE knows that any matching string must contain the letter "d". If the
! 3069: subject string is "abyz", the lack of "d" means that matching doesn't
! 3070: ever start, and the callout is never reached. However, with "abyd",
! 3071: though the result is still no match, the callout is obeyed.
! 3072:
! 3073: If the pattern is studied, PCRE knows the minimum length of a matching
! 3074: string, and will immediately give a "no match" return without actually
! 3075: running a match if the subject is not long enough, or, for unanchored
! 3076: patterns, if it has been scanned far enough.
! 3077:
! 3078: You can disable these optimizations by passing the PCRE_NO_START_OPTI-
! 3079: MIZE option to pcre_compile(), pcre_exec(), or pcre_dfa_exec(), or by
! 3080: starting the pattern with (*NO_START_OPT). This slows down the matching
! 3081: process, but does ensure that callouts such as the example above are
! 3082: obeyed.
! 3083:
! 3084:
! 3085: THE CALLOUT INTERFACE
! 3086:
! 3087: During matching, when PCRE reaches a callout point, the external func-
! 3088: tion defined by pcre_callout is called (if it is set). This applies to
! 3089: both the pcre_exec() and the pcre_dfa_exec() matching functions. The
! 3090: only argument to the callout function is a pointer to a pcre_callout
! 3091: block. This structure contains the following fields:
! 3092:
! 3093: int version;
! 3094: int callout_number;
! 3095: int *offset_vector;
! 3096: const char *subject;
! 3097: int subject_length;
! 3098: int start_match;
! 3099: int current_position;
! 3100: int capture_top;
! 3101: int capture_last;
! 3102: void *callout_data;
! 3103: int pattern_position;
! 3104: int next_item_length;
! 3105: const unsigned char *mark;
! 3106:
! 3107: The version field is an integer containing the version number of the
! 3108: block format. The initial version was 0; the current version is 2. The
! 3109: version number will change again in future if additional fields are
! 3110: added, but the intention is never to remove any of the existing fields.
! 3111:
! 3112: The callout_number field contains the number of the callout, as com-
! 3113: piled into the pattern (that is, the number after ?C for manual call-
! 3114: outs, and 255 for automatically generated callouts).
! 3115:
! 3116: The offset_vector field is a pointer to the vector of offsets that was
! 3117: passed by the caller to pcre_exec() or pcre_dfa_exec(). When
! 3118: pcre_exec() is used, the contents can be inspected in order to extract
! 3119: substrings that have been matched so far, in the same way as for
! 3120: extracting substrings after a match has completed. For pcre_dfa_exec()
! 3121: this field is not useful.
! 3122:
! 3123: The subject and subject_length fields contain copies of the values that
! 3124: were passed to pcre_exec().
! 3125:
! 3126: The start_match field normally contains the offset within the subject
! 3127: at which the current match attempt started. However, if the escape
! 3128: sequence \K has been encountered, this value is changed to reflect the
! 3129: modified starting point. If the pattern is not anchored, the callout
! 3130: function may be called several times from the same point in the pattern
! 3131: for different starting points in the subject.
! 3132:
! 3133: The current_position field contains the offset within the subject of
! 3134: the current match pointer.
! 3135:
! 3136: When the pcre_exec() function is used, the capture_top field contains
! 3137: one more than the number of the highest numbered captured substring so
! 3138: far. If no substrings have been captured, the value of capture_top is
! 3139: one. This is always the case when pcre_dfa_exec() is used, because it
! 3140: does not support captured substrings.
! 3141:
! 3142: The capture_last field contains the number of the most recently cap-
! 3143: tured substring. If no substrings have been captured, its value is -1.
! 3144: This is always the case when pcre_dfa_exec() is used.
! 3145:
! 3146: The callout_data field contains a value that is passed to pcre_exec()
! 3147: or pcre_dfa_exec() specifically so that it can be passed back in call-
! 3148: outs. It is passed in the pcre_callout field of the pcre_extra data
! 3149: structure. If no such data was passed, the value of callout_data in a
! 3150: pcre_callout block is NULL. There is a description of the pcre_extra
! 3151: structure in the pcreapi documentation.
! 3152:
! 3153: The pattern_position field is present from version 1 of the pcre_call-
! 3154: out structure. It contains the offset to the next item to be matched in
! 3155: the pattern string.
! 3156:
! 3157: The next_item_length field is present from version 1 of the pcre_call-
! 3158: out structure. It contains the length of the next item to be matched in
! 3159: the pattern string. When the callout immediately precedes an alterna-
! 3160: tion bar, a closing parenthesis, or the end of the pattern, the length
! 3161: is zero. When the callout precedes an opening parenthesis, the length
! 3162: is that of the entire subpattern.
! 3163:
! 3164: The pattern_position and next_item_length fields are intended to help
! 3165: in distinguishing between different automatic callouts, which all have
! 3166: the same callout number. However, they are set for all callouts.
! 3167:
! 3168: The mark field is present from version 2 of the pcre_callout structure.
! 3169: In callouts from pcre_exec() it contains a pointer to the zero-termi-
! 3170: nated name of the most recently passed (*MARK), (*PRUNE), or (*THEN)
! 3171: item in the match, or NULL if no such items have been passed. Instances
! 3172: of (*PRUNE) or (*THEN) without a name do not obliterate a previous
! 3173: (*MARK). In callouts from pcre_dfa_exec() this field always contains
! 3174: NULL.
! 3175:
! 3176:
! 3177: RETURN VALUES
! 3178:
! 3179: The external callout function returns an integer to PCRE. If the value
! 3180: is zero, matching proceeds as normal. If the value is greater than
! 3181: zero, matching fails at the current point, but the testing of other
! 3182: matching possibilities goes ahead, just as if a lookahead assertion had
! 3183: failed. If the value is less than zero, the match is abandoned, and
! 3184: pcre_exec() or pcre_dfa_exec() returns the negative value.
! 3185:
! 3186: Negative values should normally be chosen from the set of
! 3187: PCRE_ERROR_xxx values. In particular, PCRE_ERROR_NOMATCH forces a stan-
! 3188: dard "no match" failure. The error number PCRE_ERROR_CALLOUT is
! 3189: reserved for use by callout functions; it will never be used by PCRE
! 3190: itself.
! 3191:
! 3192:
! 3193: AUTHOR
! 3194:
! 3195: Philip Hazel
! 3196: University Computing Service
! 3197: Cambridge CB2 3QH, England.
! 3198:
! 3199:
! 3200: REVISION
! 3201:
! 3202: Last updated: 30 November 2011
! 3203: Copyright (c) 1997-2011 University of Cambridge.
! 3204: ------------------------------------------------------------------------------
! 3205:
! 3206:
! 3207: PCRECOMPAT(3) PCRECOMPAT(3)
! 3208:
! 3209:
! 3210: NAME
! 3211: PCRE - Perl-compatible regular expressions
! 3212:
! 3213:
! 3214: DIFFERENCES BETWEEN PCRE AND PERL
! 3215:
! 3216: This document describes the differences in the ways that PCRE and Perl
! 3217: handle regular expressions. The differences described here are with
! 3218: respect to Perl versions 5.10 and above.
! 3219:
! 3220: 1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
! 3221: of what it does have are given in the pcreunicode page.
! 3222:
! 3223: 2. PCRE allows repeat quantifiers only on parenthesized assertions, but
! 3224: they do not mean what you might think. For example, (?!a){3} does not
! 3225: assert that the next three characters are not "a". It just asserts that
! 3226: the next character is not "a" three times (in principle: PCRE optimizes
! 3227: this to run the assertion just once). Perl allows repeat quantifiers on
! 3228: other assertions such as \b, but these do not seem to have any use.
! 3229:
! 3230: 3. Capturing subpatterns that occur inside negative lookahead asser-
! 3231: tions are counted, but their entries in the offsets vector are never
! 3232: set. Perl sets its numerical variables from any such patterns that are
! 3233: matched before the assertion fails to match something (thereby succeed-
! 3234: ing), but only if the negative lookahead assertion contains just one
! 3235: branch.
! 3236:
! 3237: 4. Though binary zero characters are supported in the subject string,
! 3238: they are not allowed in a pattern string because it is passed as a nor-
! 3239: mal C string, terminated by zero. The escape sequence \0 can be used in
! 3240: the pattern to represent a binary zero.
! 3241:
! 3242: 5. The following Perl escape sequences are not supported: \l, \u, \L,
! 3243: \U, and \N when followed by a character name or Unicode value. (\N on
! 3244: its own, matching a non-newline character, is supported.) In fact these
! 3245: are implemented by Perl's general string-handling and are not part of
! 3246: its pattern matching engine. If any of these are encountered by PCRE,
! 3247: an error is generated by default. However, if the PCRE_JAVASCRIPT_COM-
! 3248: PAT option is set, \U and \u are interpreted as JavaScript interprets
! 3249: them.
! 3250:
! 3251: 6. The Perl escape sequences \p, \P, and \X are supported only if PCRE
! 3252: is built with Unicode character property support. The properties that
! 3253: can be tested with \p and \P are limited to the general category prop-
! 3254: erties such as Lu and Nd, script names such as Greek or Han, and the
! 3255: derived properties Any and L&. PCRE does support the Cs (surrogate)
! 3256: property, which Perl does not; the Perl documentation says "Because
! 3257: Perl hides the need for the user to understand the internal representa-
! 3258: tion of Unicode characters, there is no need to implement the somewhat
! 3259: messy concept of surrogates."
! 3260:
! 3261: 7. PCRE implements a simpler version of \X than Perl, which changed to
! 3262: make \X match what Unicode calls an "extended grapheme cluster". This
! 3263: is more complicated than an extended Unicode sequence, which is what
! 3264: PCRE matches.
! 3265:
! 3266: 8. PCRE does support the \Q...\E escape for quoting substrings. Charac-
! 3267: ters in between are treated as literals. This is slightly different
! 3268: from Perl in that $ and @ are also handled as literals inside the
! 3269: quotes. In Perl, they cause variable interpolation (but of course PCRE
! 3270: does not have variables). Note the following examples:
! 3271:
! 3272: Pattern PCRE matches Perl matches
! 3273:
! 3274: \Qabc$xyz\E abc$xyz abc followed by the
! 3275: contents of $xyz
! 3276: \Qabc\$xyz\E abc\$xyz abc\$xyz
! 3277: \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
! 3278:
! 3279: The \Q...\E sequence is recognized both inside and outside character
! 3280: classes.
! 3281:
! 3282: 9. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
! 3283: constructions. However, there is support for recursive patterns. This
! 3284: is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE
! 3285: "callout" feature allows an external function to be called during pat-
! 3286: tern matching. See the pcrecallout documentation for details.
! 3287:
! 3288: 10. Subpatterns that are called as subroutines (whether or not recur-
! 3289: sively) are always treated as atomic groups in PCRE. This is like
! 3290: Python, but unlike Perl. Captured values that are set outside a sub-
! 3291: routine call can be reference from inside in PCRE, but not in Perl.
! 3292: There is a discussion that explains these differences in more detail in
! 3293: the section on recursion differences from Perl in the pcrepattern page.
! 3294:
! 3295: 11. If (*THEN) is present in a group that is called as a subroutine,
! 3296: its action is limited to that group, even if the group does not contain
! 3297: any | characters.
! 3298:
! 3299: 12. There are some differences that are concerned with the settings of
! 3300: captured strings when part of a pattern is repeated. For example,
! 3301: matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
! 3302: unset, but in PCRE it is set to "b".
! 3303:
! 3304: 13. PCRE's handling of duplicate subpattern numbers and duplicate sub-
! 3305: pattern names is not as general as Perl's. This is a consequence of the
! 3306: fact the PCRE works internally just with numbers, using an external ta-
! 3307: ble to translate between numbers and names. In particular, a pattern
! 3308: such as (?|(?<a>A)|(?<b)B), where the two capturing parentheses have
! 3309: the same number but different names, is not supported, and causes an
! 3310: error at compile time. If it were allowed, it would not be possible to
! 3311: distinguish which parentheses matched, because both names map to cap-
! 3312: turing subpattern number 1. To avoid this confusing situation, an error
! 3313: is given at compile time.
! 3314:
! 3315: 14. Perl recognizes comments in some places that PCRE does not, for
! 3316: example, between the ( and ? at the start of a subpattern. If the /x
! 3317: modifier is set, Perl allows whitespace between ( and ? but PCRE never
! 3318: does, even if the PCRE_EXTENDED option is set.
! 3319:
! 3320: 15. PCRE provides some extensions to the Perl regular expression facil-
! 3321: ities. Perl 5.10 includes new features that are not in earlier ver-
! 3322: sions of Perl, some of which (such as named parentheses) have been in
! 3323: PCRE for some time. This list is with respect to Perl 5.10:
! 3324:
! 3325: (a) Although lookbehind assertions in PCRE must match fixed length
! 3326: strings, each alternative branch of a lookbehind assertion can match a
! 3327: different length of string. Perl requires them all to have the same
! 3328: length.
! 3329:
! 3330: (b) If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
! 3331: meta-character matches only at the very end of the string.
! 3332:
! 3333: (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe-
! 3334: cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
! 3335: ignored. (Perl can be made to issue a warning.)
! 3336:
! 3337: (d) If PCRE_UNGREEDY is set, the greediness of the repetition quanti-
! 3338: fiers is inverted, that is, by default they are not greedy, but if fol-
! 3339: lowed by a question mark they are.
! 3340:
! 3341: (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
! 3342: tried only at the first matching position in the subject string.
! 3343:
! 3344: (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
! 3345: and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no Perl equiva-
! 3346: lents.
! 3347:
! 3348: (g) The \R escape sequence can be restricted to match only CR, LF, or
! 3349: CRLF by the PCRE_BSR_ANYCRLF option.
! 3350:
! 3351: (h) The callout facility is PCRE-specific.
! 3352:
! 3353: (i) The partial matching facility is PCRE-specific.
! 3354:
! 3355: (j) Patterns compiled by PCRE can be saved and re-used at a later time,
! 3356: even on different hosts that have the other endianness. However, this
! 3357: does not apply to optimized data created by the just-in-time compiler.
! 3358:
! 3359: (k) The alternative matching function (pcre_dfa_exec()) matches in a
! 3360: different way and is not Perl-compatible.
! 3361:
! 3362: (l) PCRE recognizes some special sequences such as (*CR) at the start
! 3363: of a pattern that set overall options that cannot be changed within the
! 3364: pattern.
! 3365:
! 3366:
! 3367: AUTHOR
! 3368:
! 3369: Philip Hazel
! 3370: University Computing Service
! 3371: Cambridge CB2 3QH, England.
! 3372:
! 3373:
! 3374: REVISION
! 3375:
! 3376: Last updated: 14 November 2011
! 3377: Copyright (c) 1997-2011 University of Cambridge.
! 3378: ------------------------------------------------------------------------------
! 3379:
! 3380:
! 3381: PCREPATTERN(3) PCREPATTERN(3)
! 3382:
! 3383:
! 3384: NAME
! 3385: PCRE - Perl-compatible regular expressions
! 3386:
! 3387:
! 3388: PCRE REGULAR EXPRESSION DETAILS
! 3389:
! 3390: The syntax and semantics of the regular expressions that are supported
! 3391: by PCRE are described in detail below. There is a quick-reference syn-
! 3392: tax summary in the pcresyntax page. PCRE tries to match Perl syntax and
! 3393: semantics as closely as it can. PCRE also supports some alternative
! 3394: regular expression syntax (which does not conflict with the Perl syn-
! 3395: tax) in order to provide some compatibility with regular expressions in
! 3396: Python, .NET, and Oniguruma.
! 3397:
! 3398: Perl's regular expressions are described in its own documentation, and
! 3399: regular expressions in general are covered in a number of books, some
! 3400: of which have copious examples. Jeffrey Friedl's "Mastering Regular
! 3401: Expressions", published by O'Reilly, covers regular expressions in
! 3402: great detail. This description of PCRE's regular expressions is
! 3403: intended as reference material.
! 3404:
! 3405: The original operation of PCRE was on strings of one-byte characters.
! 3406: However, there is now also support for UTF-8 character strings. To use
! 3407: this, PCRE must be built to include UTF-8 support, and you must call
! 3408: pcre_compile() or pcre_compile2() with the PCRE_UTF8 option. There is
! 3409: also a special sequence that can be given at the start of a pattern:
! 3410:
! 3411: (*UTF8)
! 3412:
! 3413: Starting a pattern with this sequence is equivalent to setting the
! 3414: PCRE_UTF8 option. This feature is not Perl-compatible. How setting
! 3415: UTF-8 mode affects pattern matching is mentioned in several places
! 3416: below. There is also a summary of UTF-8 features in the pcreunicode
! 3417: page.
! 3418:
! 3419: Another special sequence that may appear at the start of a pattern or
! 3420: in combination with (*UTF8) is:
! 3421:
! 3422: (*UCP)
! 3423:
! 3424: This has the same effect as setting the PCRE_UCP option: it causes
! 3425: sequences such as \d and \w to use Unicode properties to determine
! 3426: character types, instead of recognizing only characters with codes less
! 3427: than 128 via a lookup table.
! 3428:
! 3429: If a pattern starts with (*NO_START_OPT), it has the same effect as
! 3430: setting the PCRE_NO_START_OPTIMIZE option either at compile or matching
! 3431: time. There are also some more of these special sequences that are con-
! 3432: cerned with the handling of newlines; they are described below.
! 3433:
! 3434: The remainder of this document discusses the patterns that are sup-
! 3435: ported by PCRE when its main matching function, pcre_exec(), is used.
! 3436: From release 6.0, PCRE offers a second matching function,
! 3437: pcre_dfa_exec(), which matches using a different algorithm that is not
! 3438: Perl-compatible. Some of the features discussed below are not available
! 3439: when pcre_dfa_exec() is used. The advantages and disadvantages of the
! 3440: alternative function, and how it differs from the normal function, are
! 3441: discussed in the pcrematching page.
! 3442:
! 3443:
! 3444: NEWLINE CONVENTIONS
! 3445:
! 3446: PCRE supports five different conventions for indicating line breaks in
! 3447: strings: a single CR (carriage return) character, a single LF (line-
! 3448: feed) character, the two-character sequence CRLF, any of the three pre-
! 3449: ceding, or any Unicode newline sequence. The pcreapi page has further
! 3450: discussion about newlines, and shows how to set the newline convention
! 3451: in the options arguments for the compiling and matching functions.
! 3452:
! 3453: It is also possible to specify a newline convention by starting a pat-
! 3454: tern string with one of the following five sequences:
! 3455:
! 3456: (*CR) carriage return
! 3457: (*LF) linefeed
! 3458: (*CRLF) carriage return, followed by linefeed
! 3459: (*ANYCRLF) any of the three above
! 3460: (*ANY) all Unicode newline sequences
! 3461:
! 3462: These override the default and the options given to pcre_compile() or
! 3463: pcre_compile2(). For example, on a Unix system where LF is the default
! 3464: newline sequence, the pattern
! 3465:
! 3466: (*CR)a.b
! 3467:
! 3468: changes the convention to CR. That pattern matches "a\nb" because LF is
! 3469: no longer a newline. Note that these special settings, which are not
! 3470: Perl-compatible, are recognized only at the very start of a pattern,
! 3471: and that they must be in upper case. If more than one of them is
! 3472: present, the last one is used.
! 3473:
! 3474: The newline convention affects the interpretation of the dot metachar-
! 3475: acter when PCRE_DOTALL is not set, and also the behaviour of \N. How-
! 3476: ever, it does not affect what the \R escape sequence matches. By
! 3477: default, this is any Unicode newline sequence, for Perl compatibility.
! 3478: However, this can be changed; see the description of \R in the section
! 3479: entitled "Newline sequences" below. A change of \R setting can be com-
! 3480: bined with a change of newline convention.
! 3481:
! 3482:
! 3483: CHARACTERS AND METACHARACTERS
! 3484:
! 3485: A regular expression is a pattern that is matched against a subject
! 3486: string from left to right. Most characters stand for themselves in a
! 3487: pattern, and match the corresponding characters in the subject. As a
! 3488: trivial example, the pattern
! 3489:
! 3490: The quick brown fox
! 3491:
! 3492: matches a portion of a subject string that is identical to itself. When
! 3493: caseless matching is specified (the PCRE_CASELESS option), letters are
! 3494: matched independently of case. In UTF-8 mode, PCRE always understands
! 3495: the concept of case for characters whose values are less than 128, so
! 3496: caseless matching is always possible. For characters with higher val-
! 3497: ues, the concept of case is supported if PCRE is compiled with Unicode
! 3498: property support, but not otherwise. If you want to use caseless
! 3499: matching for characters 128 and above, you must ensure that PCRE is
! 3500: compiled with Unicode property support as well as with UTF-8 support.
! 3501:
! 3502: The power of regular expressions comes from the ability to include
! 3503: alternatives and repetitions in the pattern. These are encoded in the
! 3504: pattern by the use of metacharacters, which do not stand for themselves
! 3505: but instead are interpreted in some special way.
! 3506:
! 3507: There are two different sets of metacharacters: those that are recog-
! 3508: nized anywhere in the pattern except within square brackets, and those
! 3509: that are recognized within square brackets. Outside square brackets,
! 3510: the metacharacters are as follows:
! 3511:
! 3512: \ general escape character with several uses
! 3513: ^ assert start of string (or line, in multiline mode)
! 3514: $ assert end of string (or line, in multiline mode)
! 3515: . match any character except newline (by default)
! 3516: [ start character class definition
! 3517: | start of alternative branch
! 3518: ( start subpattern
! 3519: ) end subpattern
! 3520: ? extends the meaning of (
! 3521: also 0 or 1 quantifier
! 3522: also quantifier minimizer
! 3523: * 0 or more quantifier
! 3524: + 1 or more quantifier
! 3525: also "possessive quantifier"
! 3526: { start min/max quantifier
! 3527:
! 3528: Part of a pattern that is in square brackets is called a "character
! 3529: class". In a character class the only metacharacters are:
! 3530:
! 3531: \ general escape character
! 3532: ^ negate the class, but only if the first character
! 3533: - indicates character range
! 3534: [ POSIX character class (only if followed by POSIX
! 3535: syntax)
! 3536: ] terminates the character class
! 3537:
! 3538: The following sections describe the use of each of the metacharacters.
! 3539:
! 3540:
! 3541: BACKSLASH
! 3542:
! 3543: The backslash character has several uses. Firstly, if it is followed by
! 3544: a character that is not a number or a letter, it takes away any special
! 3545: meaning that character may have. This use of backslash as an escape
! 3546: character applies both inside and outside character classes.
! 3547:
! 3548: For example, if you want to match a * character, you write \* in the
! 3549: pattern. This escaping action applies whether or not the following
! 3550: character would otherwise be interpreted as a metacharacter, so it is
! 3551: always safe to precede a non-alphanumeric with backslash to specify
! 3552: that it stands for itself. In particular, if you want to match a back-
! 3553: slash, you write \\.
! 3554:
! 3555: In UTF-8 mode, only ASCII numbers and letters have any special meaning
! 3556: after a backslash. All other characters (in particular, those whose
! 3557: codepoints are greater than 127) are treated as literals.
! 3558:
! 3559: If a pattern is compiled with the PCRE_EXTENDED option, whitespace in
! 3560: the pattern (other than in a character class) and characters between a
! 3561: # outside a character class and the next newline are ignored. An escap-
! 3562: ing backslash can be used to include a whitespace or # character as
! 3563: part of the pattern.
! 3564:
! 3565: If you want to remove the special meaning from a sequence of charac-
! 3566: ters, you can do so by putting them between \Q and \E. This is differ-
! 3567: ent from Perl in that $ and @ are handled as literals in \Q...\E
! 3568: sequences in PCRE, whereas in Perl, $ and @ cause variable interpola-
! 3569: tion. Note the following examples:
! 3570:
! 3571: Pattern PCRE matches Perl matches
! 3572:
! 3573: \Qabc$xyz\E abc$xyz abc followed by the
! 3574: contents of $xyz
! 3575: \Qabc\$xyz\E abc\$xyz abc\$xyz
! 3576: \Qabc\E\$\Qxyz\E abc$xyz abc$xyz
! 3577:
! 3578: The \Q...\E sequence is recognized both inside and outside character
! 3579: classes. An isolated \E that is not preceded by \Q is ignored. If \Q
! 3580: is not followed by \E later in the pattern, the literal interpretation
! 3581: continues to the end of the pattern (that is, \E is assumed at the
! 3582: end). If the isolated \Q is inside a character class, this causes an
! 3583: error, because the character class is not terminated.
! 3584:
! 3585: Non-printing characters
! 3586:
! 3587: A second use of backslash provides a way of encoding non-printing char-
! 3588: acters in patterns in a visible manner. There is no restriction on the
! 3589: appearance of non-printing characters, apart from the binary zero that
! 3590: terminates a pattern, but when a pattern is being prepared by text
! 3591: editing, it is often easier to use one of the following escape
! 3592: sequences than the binary character it represents:
! 3593:
! 3594: \a alarm, that is, the BEL character (hex 07)
! 3595: \cx "control-x", where x is any ASCII character
! 3596: \e escape (hex 1B)
! 3597: \f formfeed (hex 0C)
! 3598: \n linefeed (hex 0A)
! 3599: \r carriage return (hex 0D)
! 3600: \t tab (hex 09)
! 3601: \ddd character with octal code ddd, or back reference
! 3602: \xhh character with hex code hh
! 3603: \x{hhh..} character with hex code hhh.. (non-JavaScript mode)
! 3604: \uhhhh character with hex code hhhh (JavaScript mode only)
! 3605:
! 3606: The precise effect of \cx is as follows: if x is a lower case letter,
! 3607: it is converted to upper case. Then bit 6 of the character (hex 40) is
! 3608: inverted. Thus \cz becomes hex 1A (z is 7A), but \c{ becomes hex 3B ({
! 3609: is 7B), while \c; becomes hex 7B (; is 3B). If the byte following \c
! 3610: has a value greater than 127, a compile-time error occurs. This locks
! 3611: out non-ASCII characters in both byte mode and UTF-8 mode. (When PCRE
! 3612: is compiled in EBCDIC mode, all byte values are valid. A lower case
! 3613: letter is converted to upper case, and then the 0xc0 bits are flipped.)
! 3614:
! 3615: By default, after \x, from zero to two hexadecimal digits are read
! 3616: (letters can be in upper or lower case). Any number of hexadecimal dig-
! 3617: its may appear between \x{ and }, but the value of the character code
! 3618: must be less than 256 in non-UTF-8 mode, and less than 2**31 in UTF-8
! 3619: mode. That is, the maximum value in hexadecimal is 7FFFFFFF. Note that
! 3620: this is bigger than the largest Unicode code point, which is 10FFFF.
! 3621:
! 3622: If characters other than hexadecimal digits appear between \x{ and },
! 3623: or if there is no terminating }, this form of escape is not recognized.
! 3624: Instead, the initial \x will be interpreted as a basic hexadecimal
! 3625: escape, with no following digits, giving a character whose value is
! 3626: zero.
! 3627:
! 3628: If the PCRE_JAVASCRIPT_COMPAT option is set, the interpretation of \x
! 3629: is as just described only when it is followed by two hexadecimal dig-
! 3630: its. Otherwise, it matches a literal "x" character. In JavaScript
! 3631: mode, support for code points greater than 256 is provided by \u, which
! 3632: must be followed by four hexadecimal digits; otherwise it matches a
! 3633: literal "u" character.
! 3634:
! 3635: Characters whose value is less than 256 can be defined by either of the
! 3636: two syntaxes for \x (or by \u in JavaScript mode). There is no differ-
! 3637: ence in the way they are handled. For example, \xdc is exactly the same
! 3638: as \x{dc} (or \u00dc in JavaScript mode).
! 3639:
! 3640: After \0 up to two further octal digits are read. If there are fewer
! 3641: than two digits, just those that are present are used. Thus the
! 3642: sequence \0\x\07 specifies two binary zeros followed by a BEL character
! 3643: (code value 7). Make sure you supply two digits after the initial zero
! 3644: if the pattern character that follows is itself an octal digit.
! 3645:
! 3646: The handling of a backslash followed by a digit other than 0 is compli-
! 3647: cated. Outside a character class, PCRE reads it and any following dig-
! 3648: its as a decimal number. If the number is less than 10, or if there
! 3649: have been at least that many previous capturing left parentheses in the
! 3650: expression, the entire sequence is taken as a back reference. A
! 3651: description of how this works is given later, following the discussion
! 3652: of parenthesized subpatterns.
! 3653:
! 3654: Inside a character class, or if the decimal number is greater than 9
! 3655: and there have not been that many capturing subpatterns, PCRE re-reads
! 3656: up to three octal digits following the backslash, and uses them to gen-
! 3657: erate a data character. Any subsequent digits stand for themselves. In
! 3658: non-UTF-8 mode, the value of a character specified in octal must be
! 3659: less than \400. In UTF-8 mode, values up to \777 are permitted. For
! 3660: example:
! 3661:
! 3662: \040 is another way of writing a space
! 3663: \40 is the same, provided there are fewer than 40
! 3664: previous capturing subpatterns
! 3665: \7 is always a back reference
! 3666: \11 might be a back reference, or another way of
! 3667: writing a tab
! 3668: \011 is always a tab
! 3669: \0113 is a tab followed by the character "3"
! 3670: \113 might be a back reference, otherwise the
! 3671: character with octal code 113
! 3672: \377 might be a back reference, otherwise
! 3673: the byte consisting entirely of 1 bits
! 3674: \81 is either a back reference, or a binary zero
! 3675: followed by the two characters "8" and "1"
! 3676:
! 3677: Note that octal values of 100 or greater must not be introduced by a
! 3678: leading zero, because no more than three octal digits are ever read.
! 3679:
! 3680: All the sequences that define a single character value can be used both
! 3681: inside and outside character classes. In addition, inside a character
! 3682: class, \b is interpreted as the backspace character (hex 08).
! 3683:
! 3684: \N is not allowed in a character class. \B, \R, and \X are not special
! 3685: inside a character class. Like other unrecognized escape sequences,
! 3686: they are treated as the literal characters "B", "R", and "X" by
! 3687: default, but cause an error if the PCRE_EXTRA option is set. Outside a
! 3688: character class, these sequences have different meanings.
! 3689:
! 3690: Unsupported escape sequences
! 3691:
! 3692: In Perl, the sequences \l, \L, \u, and \U are recognized by its string
! 3693: handler and used to modify the case of following characters. By
! 3694: default, PCRE does not support these escape sequences. However, if the
! 3695: PCRE_JAVASCRIPT_COMPAT option is set, \U matches a "U" character, and
! 3696: \u can be used to define a character by code point, as described in the
! 3697: previous section.
! 3698:
! 3699: Absolute and relative back references
! 3700:
! 3701: The sequence \g followed by an unsigned or a negative number, option-
! 3702: ally enclosed in braces, is an absolute or relative back reference. A
! 3703: named back reference can be coded as \g{name}. Back references are dis-
! 3704: cussed later, following the discussion of parenthesized subpatterns.
! 3705:
! 3706: Absolute and relative subroutine calls
! 3707:
! 3708: For compatibility with Oniguruma, the non-Perl syntax \g followed by a
! 3709: name or a number enclosed either in angle brackets or single quotes, is
! 3710: an alternative syntax for referencing a subpattern as a "subroutine".
! 3711: Details are discussed later. Note that \g{...} (Perl syntax) and
! 3712: \g<...> (Oniguruma syntax) are not synonymous. The former is a back
! 3713: reference; the latter is a subroutine call.
! 3714:
! 3715: Generic character types
! 3716:
! 3717: Another use of backslash is for specifying generic character types:
! 3718:
! 3719: \d any decimal digit
! 3720: \D any character that is not a decimal digit
! 3721: \h any horizontal whitespace character
! 3722: \H any character that is not a horizontal whitespace character
! 3723: \s any whitespace character
! 3724: \S any character that is not a whitespace character
! 3725: \v any vertical whitespace character
! 3726: \V any character that is not a vertical whitespace character
! 3727: \w any "word" character
! 3728: \W any "non-word" character
! 3729:
! 3730: There is also the single sequence \N, which matches a non-newline char-
! 3731: acter. This is the same as the "." metacharacter when PCRE_DOTALL is
! 3732: not set. Perl also uses \N to match characters by name; PCRE does not
! 3733: support this.
! 3734:
! 3735: Each pair of lower and upper case escape sequences partitions the com-
! 3736: plete set of characters into two disjoint sets. Any given character
! 3737: matches one, and only one, of each pair. The sequences can appear both
! 3738: inside and outside character classes. They each match one character of
! 3739: the appropriate type. If the current matching point is at the end of
! 3740: the subject string, all of them fail, because there is no character to
! 3741: match.
! 3742:
! 3743: For compatibility with Perl, \s does not match the VT character (code
! 3744: 11). This makes it different from the the POSIX "space" class. The \s
! 3745: characters are HT (9), LF (10), FF (12), CR (13), and space (32). If
! 3746: "use locale;" is included in a Perl script, \s may match the VT charac-
! 3747: ter. In PCRE, it never does.
! 3748:
! 3749: A "word" character is an underscore or any character that is a letter
! 3750: or digit. By default, the definition of letters and digits is con-
! 3751: trolled by PCRE's low-valued character tables, and may vary if locale-
! 3752: specific matching is taking place (see "Locale support" in the pcreapi
! 3753: page). For example, in a French locale such as "fr_FR" in Unix-like
! 3754: systems, or "french" in Windows, some character codes greater than 128
! 3755: are used for accented letters, and these are then matched by \w. The
! 3756: use of locales with Unicode is discouraged.
! 3757:
! 3758: By default, in UTF-8 mode, characters with values greater than 128
! 3759: never match \d, \s, or \w, and always match \D, \S, and \W. These
! 3760: sequences retain their original meanings from before UTF-8 support was
! 3761: available, mainly for efficiency reasons. However, if PCRE is compiled
! 3762: with Unicode property support, and the PCRE_UCP option is set, the be-
! 3763: haviour is changed so that Unicode properties are used to determine
! 3764: character types, as follows:
! 3765:
! 3766: \d any character that \p{Nd} matches (decimal digit)
! 3767: \s any character that \p{Z} matches, plus HT, LF, FF, CR
! 3768: \w any character that \p{L} or \p{N} matches, plus underscore
! 3769:
! 3770: The upper case escapes match the inverse sets of characters. Note that
! 3771: \d matches only decimal digits, whereas \w matches any Unicode digit,
! 3772: as well as any Unicode letter, and underscore. Note also that PCRE_UCP
! 3773: affects \b, and \B because they are defined in terms of \w and \W.
! 3774: Matching these sequences is noticeably slower when PCRE_UCP is set.
! 3775:
! 3776: The sequences \h, \H, \v, and \V are features that were added to Perl
! 3777: at release 5.10. In contrast to the other sequences, which match only
! 3778: ASCII characters by default, these always match certain high-valued
! 3779: codepoints in UTF-8 mode, whether or not PCRE_UCP is set. The horizon-
! 3780: tal space characters are:
! 3781:
! 3782: U+0009 Horizontal tab
! 3783: U+0020 Space
! 3784: U+00A0 Non-break space
! 3785: U+1680 Ogham space mark
! 3786: U+180E Mongolian vowel separator
! 3787: U+2000 En quad
! 3788: U+2001 Em quad
! 3789: U+2002 En space
! 3790: U+2003 Em space
! 3791: U+2004 Three-per-em space
! 3792: U+2005 Four-per-em space
! 3793: U+2006 Six-per-em space
! 3794: U+2007 Figure space
! 3795: U+2008 Punctuation space
! 3796: U+2009 Thin space
! 3797: U+200A Hair space
! 3798: U+202F Narrow no-break space
! 3799: U+205F Medium mathematical space
! 3800: U+3000 Ideographic space
! 3801:
! 3802: The vertical space characters are:
! 3803:
! 3804: U+000A Linefeed
! 3805: U+000B Vertical tab
! 3806: U+000C Formfeed
! 3807: U+000D Carriage return
! 3808: U+0085 Next line
! 3809: U+2028 Line separator
! 3810: U+2029 Paragraph separator
! 3811:
! 3812: Newline sequences
! 3813:
! 3814: Outside a character class, by default, the escape sequence \R matches
! 3815: any Unicode newline sequence. In non-UTF-8 mode \R is equivalent to the
! 3816: following:
! 3817:
! 3818: (?>\r\n|\n|\x0b|\f|\r|\x85)
! 3819:
! 3820: This is an example of an "atomic group", details of which are given
! 3821: below. This particular group matches either the two-character sequence
! 3822: CR followed by LF, or one of the single characters LF (linefeed,
! 3823: U+000A), VT (vertical tab, U+000B), FF (formfeed, U+000C), CR (carriage
! 3824: return, U+000D), or NEL (next line, U+0085). The two-character sequence
! 3825: is treated as a single unit that cannot be split.
! 3826:
! 3827: In UTF-8 mode, two additional characters whose codepoints are greater
! 3828: than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
! 3829: rator, U+2029). Unicode character property support is not needed for
! 3830: these characters to be recognized.
! 3831:
! 3832: It is possible to restrict \R to match only CR, LF, or CRLF (instead of
! 3833: the complete set of Unicode line endings) by setting the option
! 3834: PCRE_BSR_ANYCRLF either at compile time or when the pattern is matched.
! 3835: (BSR is an abbrevation for "backslash R".) This can be made the default
! 3836: when PCRE is built; if this is the case, the other behaviour can be
! 3837: requested via the PCRE_BSR_UNICODE option. It is also possible to
! 3838: specify these settings by starting a pattern string with one of the
! 3839: following sequences:
! 3840:
! 3841: (*BSR_ANYCRLF) CR, LF, or CRLF only
! 3842: (*BSR_UNICODE) any Unicode newline sequence
! 3843:
! 3844: These override the default and the options given to pcre_compile() or
! 3845: pcre_compile2(), but they can be overridden by options given to
! 3846: pcre_exec() or pcre_dfa_exec(). Note that these special settings, which
! 3847: are not Perl-compatible, are recognized only at the very start of a
! 3848: pattern, and that they must be in upper case. If more than one of them
! 3849: is present, the last one is used. They can be combined with a change of
! 3850: newline convention; for example, a pattern can start with:
! 3851:
! 3852: (*ANY)(*BSR_ANYCRLF)
! 3853:
! 3854: They can also be combined with the (*UTF8) or (*UCP) special sequences.
! 3855: Inside a character class, \R is treated as an unrecognized escape
! 3856: sequence, and so matches the letter "R" by default, but causes an error
! 3857: if PCRE_EXTRA is set.
! 3858:
! 3859: Unicode character properties
! 3860:
! 3861: When PCRE is built with Unicode character property support, three addi-
! 3862: tional escape sequences that match characters with specific properties
! 3863: are available. When not in UTF-8 mode, these sequences are of course
! 3864: limited to testing characters whose codepoints are less than 256, but
! 3865: they do work in this mode. The extra escape sequences are:
! 3866:
! 3867: \p{xx} a character with the xx property
! 3868: \P{xx} a character without the xx property
! 3869: \X an extended Unicode sequence
! 3870:
! 3871: The property names represented by xx above are limited to the Unicode
! 3872: script names, the general category properties, "Any", which matches any
! 3873: character (including newline), and some special PCRE properties
! 3874: (described in the next section). Other Perl properties such as "InMu-
! 3875: sicalSymbols" are not currently supported by PCRE. Note that \P{Any}
! 3876: does not match any characters, so always causes a match failure.
! 3877:
! 3878: Sets of Unicode characters are defined as belonging to certain scripts.
! 3879: A character from one of these sets can be matched using a script name.
! 3880: For example:
! 3881:
! 3882: \p{Greek}
! 3883: \P{Han}
! 3884:
! 3885: Those that are not part of an identified script are lumped together as
! 3886: "Common". The current list of scripts is:
! 3887:
! 3888: Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
! 3889: Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common,
! 3890: Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp-
! 3891: tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek,
! 3892: Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe-
! 3893: rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
! 3894: Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
! 3895: Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam,
! 3896: Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
! 3897: Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya,
! 3898: Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian,
! 3899: Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le,
! 3900: Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh,
! 3901: Ugaritic, Vai, Yi.
! 3902:
! 3903: Each character has exactly one Unicode general category property, spec-
! 3904: ified by a two-letter abbreviation. For compatibility with Perl, nega-
! 3905: tion can be specified by including a circumflex between the opening
! 3906: brace and the property name. For example, \p{^Lu} is the same as
! 3907: \P{Lu}.
! 3908:
! 3909: If only one letter is specified with \p or \P, it includes all the gen-
! 3910: eral category properties that start with that letter. In this case, in
! 3911: the absence of negation, the curly brackets in the escape sequence are
! 3912: optional; these two examples have the same effect:
! 3913:
! 3914: \p{L}
! 3915: \pL
! 3916:
! 3917: The following general category property codes are supported:
! 3918:
! 3919: C Other
! 3920: Cc Control
! 3921: Cf Format
! 3922: Cn Unassigned
! 3923: Co Private use
! 3924: Cs Surrogate
! 3925:
! 3926: L Letter
! 3927: Ll Lower case letter
! 3928: Lm Modifier letter
! 3929: Lo Other letter
! 3930: Lt Title case letter
! 3931: Lu Upper case letter
! 3932:
! 3933: M Mark
! 3934: Mc Spacing mark
! 3935: Me Enclosing mark
! 3936: Mn Non-spacing mark
! 3937:
! 3938: N Number
! 3939: Nd Decimal number
! 3940: Nl Letter number
! 3941: No Other number
! 3942:
! 3943: P Punctuation
! 3944: Pc Connector punctuation
! 3945: Pd Dash punctuation
! 3946: Pe Close punctuation
! 3947: Pf Final punctuation
! 3948: Pi Initial punctuation
! 3949: Po Other punctuation
! 3950: Ps Open punctuation
! 3951:
! 3952: S Symbol
! 3953: Sc Currency symbol
! 3954: Sk Modifier symbol
! 3955: Sm Mathematical symbol
! 3956: So Other symbol
! 3957:
! 3958: Z Separator
! 3959: Zl Line separator
! 3960: Zp Paragraph separator
! 3961: Zs Space separator
! 3962:
! 3963: The special property L& is also supported: it matches a character that
! 3964: has the Lu, Ll, or Lt property, in other words, a letter that is not
! 3965: classified as a modifier or "other".
! 3966:
! 3967: The Cs (Surrogate) property applies only to characters in the range
! 3968: U+D800 to U+DFFF. Such characters are not valid in UTF-8 strings (see
! 3969: RFC 3629) and so cannot be tested by PCRE, unless UTF-8 validity check-
! 3970: ing has been turned off (see the discussion of PCRE_NO_UTF8_CHECK in
! 3971: the pcreapi page). Perl does not support the Cs property.
! 3972:
! 3973: The long synonyms for property names that Perl supports (such as
! 3974: \p{Letter}) are not supported by PCRE, nor is it permitted to prefix
! 3975: any of these properties with "Is".
! 3976:
! 3977: No character that is in the Unicode table has the Cn (unassigned) prop-
! 3978: erty. Instead, this property is assumed for any code point that is not
! 3979: in the Unicode table.
! 3980:
! 3981: Specifying caseless matching does not affect these escape sequences.
! 3982: For example, \p{Lu} always matches only upper case letters.
! 3983:
! 3984: The \X escape matches any number of Unicode characters that form an
! 3985: extended Unicode sequence. \X is equivalent to
! 3986:
! 3987: (?>\PM\pM*)
! 3988:
! 3989: That is, it matches a character without the "mark" property, followed
! 3990: by zero or more characters with the "mark" property, and treats the
! 3991: sequence as an atomic group (see below). Characters with the "mark"
! 3992: property are typically accents that affect the preceding character.
! 3993: None of them have codepoints less than 256, so in non-UTF-8 mode \X
! 3994: matches any one character.
! 3995:
! 3996: Note that recent versions of Perl have changed \X to match what Unicode
! 3997: calls an "extended grapheme cluster", which has a more complicated def-
! 3998: inition.
! 3999:
! 4000: Matching characters by Unicode property is not fast, because PCRE has
! 4001: to search a structure that contains data for over fifteen thousand
! 4002: characters. That is why the traditional escape sequences such as \d and
! 4003: \w do not use Unicode properties in PCRE by default, though you can
! 4004: make them do so by setting the PCRE_UCP option for pcre_compile() or by
! 4005: starting the pattern with (*UCP).
! 4006:
! 4007: PCRE's additional properties
! 4008:
! 4009: As well as the standard Unicode properties described in the previous
! 4010: section, PCRE supports four more that make it possible to convert tra-
! 4011: ditional escape sequences such as \w and \s and POSIX character classes
! 4012: to use Unicode properties. PCRE uses these non-standard, non-Perl prop-
! 4013: erties internally when PCRE_UCP is set. They are:
! 4014:
! 4015: Xan Any alphanumeric character
! 4016: Xps Any POSIX space character
! 4017: Xsp Any Perl space character
! 4018: Xwd Any Perl "word" character
! 4019:
! 4020: Xan matches characters that have either the L (letter) or the N (num-
! 4021: ber) property. Xps matches the characters tab, linefeed, vertical tab,
! 4022: formfeed, or carriage return, and any other character that has the Z
! 4023: (separator) property. Xsp is the same as Xps, except that vertical tab
! 4024: is excluded. Xwd matches the same characters as Xan, plus underscore.
! 4025:
! 4026: Resetting the match start
! 4027:
! 4028: The escape sequence \K causes any previously matched characters not to
! 4029: be included in the final matched sequence. For example, the pattern:
! 4030:
! 4031: foo\Kbar
! 4032:
! 4033: matches "foobar", but reports that it has matched "bar". This feature
! 4034: is similar to a lookbehind assertion (described below). However, in
! 4035: this case, the part of the subject before the real match does not have
! 4036: to be of fixed length, as lookbehind assertions do. The use of \K does
! 4037: not interfere with the setting of captured substrings. For example,
! 4038: when the pattern
! 4039:
! 4040: (foo)\Kbar
! 4041:
! 4042: matches "foobar", the first substring is still set to "foo".
! 4043:
! 4044: Perl documents that the use of \K within assertions is "not well
! 4045: defined". In PCRE, \K is acted upon when it occurs inside positive
! 4046: assertions, but is ignored in negative assertions.
! 4047:
! 4048: Simple assertions
! 4049:
! 4050: The final use of backslash is for certain simple assertions. An asser-
! 4051: tion specifies a condition that has to be met at a particular point in
! 4052: a match, without consuming any characters from the subject string. The
! 4053: use of subpatterns for more complicated assertions is described below.
! 4054: The backslashed assertions are:
! 4055:
! 4056: \b matches at a word boundary
! 4057: \B matches when not at a word boundary
! 4058: \A matches at the start of the subject
! 4059: \Z matches at the end of the subject
! 4060: also matches before a newline at the end of the subject
! 4061: \z matches only at the end of the subject
! 4062: \G matches at the first matching position in the subject
! 4063:
! 4064: Inside a character class, \b has a different meaning; it matches the
! 4065: backspace character. If any other of these assertions appears in a
! 4066: character class, by default it matches the corresponding literal char-
! 4067: acter (for example, \B matches the letter B). However, if the
! 4068: PCRE_EXTRA option is set, an "invalid escape sequence" error is gener-
! 4069: ated instead.
! 4070:
! 4071: A word boundary is a position in the subject string where the current
! 4072: character and the previous character do not both match \w or \W (i.e.
! 4073: one matches \w and the other matches \W), or the start or end of the
! 4074: string if the first or last character matches \w, respectively. In
! 4075: UTF-8 mode, the meanings of \w and \W can be changed by setting the
! 4076: PCRE_UCP option. When this is done, it also affects \b and \B. Neither
! 4077: PCRE nor Perl has a separate "start of word" or "end of word" metase-
! 4078: quence. However, whatever follows \b normally determines which it is.
! 4079: For example, the fragment \ba matches "a" at the start of a word.
! 4080:
! 4081: The \A, \Z, and \z assertions differ from the traditional circumflex
! 4082: and dollar (described in the next section) in that they only ever match
! 4083: at the very start and end of the subject string, whatever options are
! 4084: set. Thus, they are independent of multiline mode. These three asser-
! 4085: tions are not affected by the PCRE_NOTBOL or PCRE_NOTEOL options, which
! 4086: affect only the behaviour of the circumflex and dollar metacharacters.
! 4087: However, if the startoffset argument of pcre_exec() is non-zero, indi-
! 4088: cating that matching is to start at a point other than the beginning of
! 4089: the subject, \A can never match. The difference between \Z and \z is
! 4090: that \Z matches before a newline at the end of the string as well as at
! 4091: the very end, whereas \z matches only at the end.
! 4092:
! 4093: The \G assertion is true only when the current matching position is at
! 4094: the start point of the match, as specified by the startoffset argument
! 4095: of pcre_exec(). It differs from \A when the value of startoffset is
! 4096: non-zero. By calling pcre_exec() multiple times with appropriate argu-
! 4097: ments, you can mimic Perl's /g option, and it is in this kind of imple-
! 4098: mentation where \G can be useful.
! 4099:
! 4100: Note, however, that PCRE's interpretation of \G, as the start of the
! 4101: current match, is subtly different from Perl's, which defines it as the
! 4102: end of the previous match. In Perl, these can be different when the
! 4103: previously matched string was empty. Because PCRE does just one match
! 4104: at a time, it cannot reproduce this behaviour.
! 4105:
! 4106: If all the alternatives of a pattern begin with \G, the expression is
! 4107: anchored to the starting match position, and the "anchored" flag is set
! 4108: in the compiled regular expression.
! 4109:
! 4110:
! 4111: CIRCUMFLEX AND DOLLAR
! 4112:
! 4113: Outside a character class, in the default matching mode, the circumflex
! 4114: character is an assertion that is true only if the current matching
! 4115: point is at the start of the subject string. If the startoffset argu-
! 4116: ment of pcre_exec() is non-zero, circumflex can never match if the
! 4117: PCRE_MULTILINE option is unset. Inside a character class, circumflex
! 4118: has an entirely different meaning (see below).
! 4119:
! 4120: Circumflex need not be the first character of the pattern if a number
! 4121: of alternatives are involved, but it should be the first thing in each
! 4122: alternative in which it appears if the pattern is ever to match that
! 4123: branch. If all possible alternatives start with a circumflex, that is,
! 4124: if the pattern is constrained to match only at the start of the sub-
! 4125: ject, it is said to be an "anchored" pattern. (There are also other
! 4126: constructs that can cause a pattern to be anchored.)
! 4127:
! 4128: A dollar character is an assertion that is true only if the current
! 4129: matching point is at the end of the subject string, or immediately
! 4130: before a newline at the end of the string (by default). Dollar need not
! 4131: be the last character of the pattern if a number of alternatives are
! 4132: involved, but it should be the last item in any branch in which it
! 4133: appears. Dollar has no special meaning in a character class.
! 4134:
! 4135: The meaning of dollar can be changed so that it matches only at the
! 4136: very end of the string, by setting the PCRE_DOLLAR_ENDONLY option at
! 4137: compile time. This does not affect the \Z assertion.
! 4138:
! 4139: The meanings of the circumflex and dollar characters are changed if the
! 4140: PCRE_MULTILINE option is set. When this is the case, a circumflex
! 4141: matches immediately after internal newlines as well as at the start of
! 4142: the subject string. It does not match after a newline that ends the
! 4143: string. A dollar matches before any newlines in the string, as well as
! 4144: at the very end, when PCRE_MULTILINE is set. When newline is specified
! 4145: as the two-character sequence CRLF, isolated CR and LF characters do
! 4146: not indicate newlines.
! 4147:
! 4148: For example, the pattern /^abc$/ matches the subject string "def\nabc"
! 4149: (where \n represents a newline) in multiline mode, but not otherwise.
! 4150: Consequently, patterns that are anchored in single line mode because
! 4151: all branches start with ^ are not anchored in multiline mode, and a
! 4152: match for circumflex is possible when the startoffset argument of
! 4153: pcre_exec() is non-zero. The PCRE_DOLLAR_ENDONLY option is ignored if
! 4154: PCRE_MULTILINE is set.
! 4155:
! 4156: Note that the sequences \A, \Z, and \z can be used to match the start
! 4157: and end of the subject in both modes, and if all branches of a pattern
! 4158: start with \A it is always anchored, whether or not PCRE_MULTILINE is
! 4159: set.
! 4160:
! 4161:
! 4162: FULL STOP (PERIOD, DOT) AND \N
! 4163:
! 4164: Outside a character class, a dot in the pattern matches any one charac-
! 4165: ter in the subject string except (by default) a character that signi-
! 4166: fies the end of a line. In UTF-8 mode, the matched character may be
! 4167: more than one byte long.
! 4168:
! 4169: When a line ending is defined as a single character, dot never matches
! 4170: that character; when the two-character sequence CRLF is used, dot does
! 4171: not match CR if it is immediately followed by LF, but otherwise it
! 4172: matches all characters (including isolated CRs and LFs). When any Uni-
! 4173: code line endings are being recognized, dot does not match CR or LF or
! 4174: any of the other line ending characters.
! 4175:
! 4176: The behaviour of dot with regard to newlines can be changed. If the
! 4177: PCRE_DOTALL option is set, a dot matches any one character, without
! 4178: exception. If the two-character sequence CRLF is present in the subject
! 4179: string, it takes two dots to match it.
! 4180:
! 4181: The handling of dot is entirely independent of the handling of circum-
! 4182: flex and dollar, the only relationship being that they both involve
! 4183: newlines. Dot has no special meaning in a character class.
! 4184:
! 4185: The escape sequence \N behaves like a dot, except that it is not
! 4186: affected by the PCRE_DOTALL option. In other words, it matches any
! 4187: character except one that signifies the end of a line. Perl also uses
! 4188: \N to match characters by name; PCRE does not support this.
! 4189:
! 4190:
! 4191: MATCHING A SINGLE BYTE
! 4192:
! 4193: Outside a character class, the escape sequence \C matches any one byte,
! 4194: both in and out of UTF-8 mode. Unlike a dot, it always matches line-
! 4195: ending characters. The feature is provided in Perl in order to match
! 4196: individual bytes in UTF-8 mode, but it is unclear how it can usefully
! 4197: be used. Because \C breaks up characters into individual bytes, match-
! 4198: ing one byte with \C in UTF-8 mode means that the rest of the string
! 4199: may start with a malformed UTF-8 character. This has undefined results,
! 4200: because PCRE assumes that it is dealing with valid UTF-8 strings (and
! 4201: by default it checks this at the start of processing unless the
! 4202: PCRE_NO_UTF8_CHECK option is used).
! 4203:
! 4204: PCRE does not allow \C to appear in lookbehind assertions (described
! 4205: below) in UTF-8 mode, because this would make it impossible to calcu-
! 4206: late the length of the lookbehind.
! 4207:
! 4208: In general, the \C escape sequence is best avoided in UTF-8 mode. How-
! 4209: ever, one way of using it that avoids the problem of malformed UTF-8
! 4210: characters is to use a lookahead to check the length of the next char-
! 4211: acter, as in this pattern (ignore white space and line breaks):
! 4212:
! 4213: (?| (?=[\x00-\x7f])(\C) |
! 4214: (?=[\x80-\x{7ff}])(\C)(\C) |
! 4215: (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
! 4216: (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))
! 4217:
! 4218: A group that starts with (?| resets the capturing parentheses numbers
! 4219: in each alternative (see "Duplicate Subpattern Numbers" below). The
! 4220: assertions at the start of each branch check the next UTF-8 character
! 4221: for values whose encoding uses 1, 2, 3, or 4 bytes, respectively. The
! 4222: character's individual bytes are then captured by the appropriate num-
! 4223: ber of groups.
! 4224:
! 4225:
! 4226: SQUARE BRACKETS AND CHARACTER CLASSES
! 4227:
! 4228: An opening square bracket introduces a character class, terminated by a
! 4229: closing square bracket. A closing square bracket on its own is not spe-
! 4230: cial by default. However, if the PCRE_JAVASCRIPT_COMPAT option is set,
! 4231: a lone closing square bracket causes a compile-time error. If a closing
! 4232: square bracket is required as a member of the class, it should be the
! 4233: first data character in the class (after an initial circumflex, if
! 4234: present) or escaped with a backslash.
! 4235:
! 4236: A character class matches a single character in the subject. In UTF-8
! 4237: mode, the character may be more than one byte long. A matched character
! 4238: must be in the set of characters defined by the class, unless the first
! 4239: character in the class definition is a circumflex, in which case the
! 4240: subject character must not be in the set defined by the class. If a
! 4241: circumflex is actually required as a member of the class, ensure it is
! 4242: not the first character, or escape it with a backslash.
! 4243:
! 4244: For example, the character class [aeiou] matches any lower case vowel,
! 4245: while [^aeiou] matches any character that is not a lower case vowel.
! 4246: Note that a circumflex is just a convenient notation for specifying the
! 4247: characters that are in the class by enumerating those that are not. A
! 4248: class that starts with a circumflex is not an assertion; it still con-
! 4249: sumes a character from the subject string, and therefore it fails if
! 4250: the current pointer is at the end of the string.
! 4251:
! 4252: In UTF-8 mode, characters with values greater than 255 can be included
! 4253: in a class as a literal string of bytes, or by using the \x{ escaping
! 4254: mechanism.
! 4255:
! 4256: When caseless matching is set, any letters in a class represent both
! 4257: their upper case and lower case versions, so for example, a caseless
! 4258: [aeiou] matches "A" as well as "a", and a caseless [^aeiou] does not
! 4259: match "A", whereas a caseful version would. In UTF-8 mode, PCRE always
! 4260: understands the concept of case for characters whose values are less
! 4261: than 128, so caseless matching is always possible. For characters with
! 4262: higher values, the concept of case is supported if PCRE is compiled
! 4263: with Unicode property support, but not otherwise. If you want to use
! 4264: caseless matching in UTF8-mode for characters 128 and above, you must
! 4265: ensure that PCRE is compiled with Unicode property support as well as
! 4266: with UTF-8 support.
! 4267:
! 4268: Characters that might indicate line breaks are never treated in any
! 4269: special way when matching character classes, whatever line-ending
! 4270: sequence is in use, and whatever setting of the PCRE_DOTALL and
! 4271: PCRE_MULTILINE options is used. A class such as [^a] always matches one
! 4272: of these characters.
! 4273:
! 4274: The minus (hyphen) character can be used to specify a range of charac-
! 4275: ters in a character class. For example, [d-m] matches any letter
! 4276: between d and m, inclusive. If a minus character is required in a
! 4277: class, it must be escaped with a backslash or appear in a position
! 4278: where it cannot be interpreted as indicating a range, typically as the
! 4279: first or last character in the class.
! 4280:
! 4281: It is not possible to have the literal character "]" as the end charac-
! 4282: ter of a range. A pattern such as [W-]46] is interpreted as a class of
! 4283: two characters ("W" and "-") followed by a literal string "46]", so it
! 4284: would match "W46]" or "-46]". However, if the "]" is escaped with a
! 4285: backslash it is interpreted as the end of range, so [W-\]46] is inter-
! 4286: preted as a class containing a range followed by two other characters.
! 4287: The octal or hexadecimal representation of "]" can also be used to end
! 4288: a range.
! 4289:
! 4290: Ranges operate in the collating sequence of character values. They can
! 4291: also be used for characters specified numerically, for example
! 4292: [\000-\037]. In UTF-8 mode, ranges can include characters whose values
! 4293: are greater than 255, for example [\x{100}-\x{2ff}].
! 4294:
! 4295: If a range that includes letters is used when caseless matching is set,
! 4296: it matches the letters in either case. For example, [W-c] is equivalent
! 4297: to [][\\^_`wxyzabc], matched caselessly, and in non-UTF-8 mode, if
! 4298: character tables for a French locale are in use, [\xc8-\xcb] matches
! 4299: accented E characters in both cases. In UTF-8 mode, PCRE supports the
! 4300: concept of case for characters with values greater than 128 only when
! 4301: it is compiled with Unicode property support.
! 4302:
! 4303: The character escape sequences \d, \D, \h, \H, \p, \P, \s, \S, \v, \V,
! 4304: \w, and \W may appear in a character class, and add the characters that
! 4305: they match to the class. For example, [\dABCDEF] matches any hexadeci-
! 4306: mal digit. In UTF-8 mode, the PCRE_UCP option affects the meanings of
! 4307: \d, \s, \w and their upper case partners, just as it does when they
! 4308: appear outside a character class, as described in the section entitled
! 4309: "Generic character types" above. The escape sequence \b has a different
! 4310: meaning inside a character class; it matches the backspace character.
! 4311: The sequences \B, \N, \R, and \X are not special inside a character
! 4312: class. Like any other unrecognized escape sequences, they are treated
! 4313: as the literal characters "B", "N", "R", and "X" by default, but cause
! 4314: an error if the PCRE_EXTRA option is set.
! 4315:
! 4316: A circumflex can conveniently be used with the upper case character
! 4317: types to specify a more restricted set of characters than the matching
! 4318: lower case type. For example, the class [^\W_] matches any letter or
! 4319: digit, but not underscore, whereas [\w] includes underscore. A positive
! 4320: character class should be read as "something OR something OR ..." and a
! 4321: negative class as "NOT something AND NOT something AND NOT ...".
! 4322:
! 4323: The only metacharacters that are recognized in character classes are
! 4324: backslash, hyphen (only where it can be interpreted as specifying a
! 4325: range), circumflex (only at the start), opening square bracket (only
! 4326: when it can be interpreted as introducing a POSIX class name - see the
! 4327: next section), and the terminating closing square bracket. However,
! 4328: escaping other non-alphanumeric characters does no harm.
! 4329:
! 4330:
! 4331: POSIX CHARACTER CLASSES
! 4332:
! 4333: Perl supports the POSIX notation for character classes. This uses names
! 4334: enclosed by [: and :] within the enclosing square brackets. PCRE also
! 4335: supports this notation. For example,
! 4336:
! 4337: [01[:alpha:]%]
! 4338:
! 4339: matches "0", "1", any alphabetic character, or "%". The supported class
! 4340: names are:
! 4341:
! 4342: alnum letters and digits
! 4343: alpha letters
! 4344: ascii character codes 0 - 127
! 4345: blank space or tab only
! 4346: cntrl control characters
! 4347: digit decimal digits (same as \d)
! 4348: graph printing characters, excluding space
! 4349: lower lower case letters
! 4350: print printing characters, including space
! 4351: punct printing characters, excluding letters and digits and space
! 4352: space white space (not quite the same as \s)
! 4353: upper upper case letters
! 4354: word "word" characters (same as \w)
! 4355: xdigit hexadecimal digits
! 4356:
! 4357: The "space" characters are HT (9), LF (10), VT (11), FF (12), CR (13),
! 4358: and space (32). Notice that this list includes the VT character (code
! 4359: 11). This makes "space" different to \s, which does not include VT (for
! 4360: Perl compatibility).
! 4361:
! 4362: The name "word" is a Perl extension, and "blank" is a GNU extension
! 4363: from Perl 5.8. Another Perl extension is negation, which is indicated
! 4364: by a ^ character after the colon. For example,
! 4365:
! 4366: [12[:^digit:]]
! 4367:
! 4368: matches "1", "2", or any non-digit. PCRE (and Perl) also recognize the
! 4369: POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
! 4370: these are not supported, and an error is given if they are encountered.
! 4371:
! 4372: By default, in UTF-8 mode, characters with values greater than 128 do
! 4373: not match any of the POSIX character classes. However, if the PCRE_UCP
! 4374: option is passed to pcre_compile(), some of the classes are changed so
! 4375: that Unicode character properties are used. This is achieved by replac-
! 4376: ing the POSIX classes by other sequences, as follows:
! 4377:
! 4378: [:alnum:] becomes \p{Xan}
! 4379: [:alpha:] becomes \p{L}
! 4380: [:blank:] becomes \h
! 4381: [:digit:] becomes \p{Nd}
! 4382: [:lower:] becomes \p{Ll}
! 4383: [:space:] becomes \p{Xps}
! 4384: [:upper:] becomes \p{Lu}
! 4385: [:word:] becomes \p{Xwd}
! 4386:
! 4387: Negated versions, such as [:^alpha:] use \P instead of \p. The other
! 4388: POSIX classes are unchanged, and match only characters with code points
! 4389: less than 128.
! 4390:
! 4391:
! 4392: VERTICAL BAR
! 4393:
! 4394: Vertical bar characters are used to separate alternative patterns. For
! 4395: example, the pattern
! 4396:
! 4397: gilbert|sullivan
! 4398:
! 4399: matches either "gilbert" or "sullivan". Any number of alternatives may
! 4400: appear, and an empty alternative is permitted (matching the empty
! 4401: string). The matching process tries each alternative in turn, from left
! 4402: to right, and the first one that succeeds is used. If the alternatives
! 4403: are within a subpattern (defined below), "succeeds" means matching the
! 4404: rest of the main pattern as well as the alternative in the subpattern.
! 4405:
! 4406:
! 4407: INTERNAL OPTION SETTING
! 4408:
! 4409: The settings of the PCRE_CASELESS, PCRE_MULTILINE, PCRE_DOTALL, and
! 4410: PCRE_EXTENDED options (which are Perl-compatible) can be changed from
! 4411: within the pattern by a sequence of Perl option letters enclosed
! 4412: between "(?" and ")". The option letters are
! 4413:
! 4414: i for PCRE_CASELESS
! 4415: m for PCRE_MULTILINE
! 4416: s for PCRE_DOTALL
! 4417: x for PCRE_EXTENDED
! 4418:
! 4419: For example, (?im) sets caseless, multiline matching. It is also possi-
! 4420: ble to unset these options by preceding the letter with a hyphen, and a
! 4421: combined setting and unsetting such as (?im-sx), which sets PCRE_CASE-
! 4422: LESS and PCRE_MULTILINE while unsetting PCRE_DOTALL and PCRE_EXTENDED,
! 4423: is also permitted. If a letter appears both before and after the
! 4424: hyphen, the option is unset.
! 4425:
! 4426: The PCRE-specific options PCRE_DUPNAMES, PCRE_UNGREEDY, and PCRE_EXTRA
! 4427: can be changed in the same way as the Perl-compatible options by using
! 4428: the characters J, U and X respectively.
! 4429:
! 4430: When one of these option changes occurs at top level (that is, not
! 4431: inside subpattern parentheses), the change applies to the remainder of
! 4432: the pattern that follows. If the change is placed right at the start of
! 4433: a pattern, PCRE extracts it into the global options (and it will there-
! 4434: fore show up in data extracted by the pcre_fullinfo() function).
! 4435:
! 4436: An option change within a subpattern (see below for a description of
! 4437: subpatterns) affects only that part of the subpattern that follows it,
! 4438: so
! 4439:
! 4440: (a(?i)b)c
! 4441:
! 4442: matches abc and aBc and no other strings (assuming PCRE_CASELESS is not
! 4443: used). By this means, options can be made to have different settings
! 4444: in different parts of the pattern. Any changes made in one alternative
! 4445: do carry on into subsequent branches within the same subpattern. For
! 4446: example,
! 4447:
! 4448: (a(?i)b|c)
! 4449:
! 4450: matches "ab", "aB", "c", and "C", even though when matching "C" the
! 4451: first branch is abandoned before the option setting. This is because
! 4452: the effects of option settings happen at compile time. There would be
! 4453: some very weird behaviour otherwise.
! 4454:
! 4455: Note: There are other PCRE-specific options that can be set by the
! 4456: application when the compile or match functions are called. In some
! 4457: cases the pattern can contain special leading sequences such as (*CRLF)
! 4458: to override what the application has set or what has been defaulted.
! 4459: Details are given in the section entitled "Newline sequences" above.
! 4460: There are also the (*UTF8) and (*UCP) leading sequences that can be
! 4461: used to set UTF-8 and Unicode property modes; they are equivalent to
! 4462: setting the PCRE_UTF8 and the PCRE_UCP options, respectively.
! 4463:
! 4464:
! 4465: SUBPATTERNS
! 4466:
! 4467: Subpatterns are delimited by parentheses (round brackets), which can be
! 4468: nested. Turning part of a pattern into a subpattern does two things:
! 4469:
! 4470: 1. It localizes a set of alternatives. For example, the pattern
! 4471:
! 4472: cat(aract|erpillar|)
! 4473:
! 4474: matches "cataract", "caterpillar", or "cat". Without the parentheses,
! 4475: it would match "cataract", "erpillar" or an empty string.
! 4476:
! 4477: 2. It sets up the subpattern as a capturing subpattern. This means
! 4478: that, when the whole pattern matches, that portion of the subject
! 4479: string that matched the subpattern is passed back to the caller via the
! 4480: ovector argument of pcre_exec(). Opening parentheses are counted from
! 4481: left to right (starting from 1) to obtain numbers for the capturing
! 4482: subpatterns. For example, if the string "the red king" is matched
! 4483: against the pattern
! 4484:
! 4485: the ((red|white) (king|queen))
! 4486:
! 4487: the captured substrings are "red king", "red", and "king", and are num-
! 4488: bered 1, 2, and 3, respectively.
! 4489:
! 4490: The fact that plain parentheses fulfil two functions is not always
! 4491: helpful. There are often times when a grouping subpattern is required
! 4492: without a capturing requirement. If an opening parenthesis is followed
! 4493: by a question mark and a colon, the subpattern does not do any captur-
! 4494: ing, and is not counted when computing the number of any subsequent
! 4495: capturing subpatterns. For example, if the string "the white queen" is
! 4496: matched against the pattern
! 4497:
! 4498: the ((?:red|white) (king|queen))
! 4499:
! 4500: the captured substrings are "white queen" and "queen", and are numbered
! 4501: 1 and 2. The maximum number of capturing subpatterns is 65535.
! 4502:
! 4503: As a convenient shorthand, if any option settings are required at the
! 4504: start of a non-capturing subpattern, the option letters may appear
! 4505: between the "?" and the ":". Thus the two patterns
! 4506:
! 4507: (?i:saturday|sunday)
! 4508: (?:(?i)saturday|sunday)
! 4509:
! 4510: match exactly the same set of strings. Because alternative branches are
! 4511: tried from left to right, and options are not reset until the end of
! 4512: the subpattern is reached, an option setting in one branch does affect
! 4513: subsequent branches, so the above patterns match "SUNDAY" as well as
! 4514: "Saturday".
! 4515:
! 4516:
! 4517: DUPLICATE SUBPATTERN NUMBERS
! 4518:
! 4519: Perl 5.10 introduced a feature whereby each alternative in a subpattern
! 4520: uses the same numbers for its capturing parentheses. Such a subpattern
! 4521: starts with (?| and is itself a non-capturing subpattern. For example,
! 4522: consider this pattern:
! 4523:
! 4524: (?|(Sat)ur|(Sun))day
! 4525:
! 4526: Because the two alternatives are inside a (?| group, both sets of cap-
! 4527: turing parentheses are numbered one. Thus, when the pattern matches,
! 4528: you can look at captured substring number one, whichever alternative
! 4529: matched. This construct is useful when you want to capture part, but
! 4530: not all, of one of a number of alternatives. Inside a (?| group, paren-
! 4531: theses are numbered as usual, but the number is reset at the start of
! 4532: each branch. The numbers of any capturing parentheses that follow the
! 4533: subpattern start after the highest number used in any branch. The fol-
! 4534: lowing example is taken from the Perl documentation. The numbers under-
! 4535: neath show in which buffer the captured content will be stored.
! 4536:
! 4537: # before ---------------branch-reset----------- after
! 4538: / ( a ) (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
! 4539: # 1 2 2 3 2 3 4
! 4540:
! 4541: A back reference to a numbered subpattern uses the most recent value
! 4542: that is set for that number by any subpattern. The following pattern
! 4543: matches "abcabc" or "defdef":
! 4544:
! 4545: /(?|(abc)|(def))\1/
! 4546:
! 4547: In contrast, a subroutine call to a numbered subpattern always refers
! 4548: to the first one in the pattern with the given number. The following
! 4549: pattern matches "abcabc" or "defabc":
! 4550:
! 4551: /(?|(abc)|(def))(?1)/
! 4552:
! 4553: If a condition test for a subpattern's having matched refers to a non-
! 4554: unique number, the test is true if any of the subpatterns of that num-
! 4555: ber have matched.
! 4556:
! 4557: An alternative approach to using this "branch reset" feature is to use
! 4558: duplicate named subpatterns, as described in the next section.
! 4559:
! 4560:
! 4561: NAMED SUBPATTERNS
! 4562:
! 4563: Identifying capturing parentheses by number is simple, but it can be
! 4564: very hard to keep track of the numbers in complicated regular expres-
! 4565: sions. Furthermore, if an expression is modified, the numbers may
! 4566: change. To help with this difficulty, PCRE supports the naming of sub-
! 4567: patterns. This feature was not added to Perl until release 5.10. Python
! 4568: had the feature earlier, and PCRE introduced it at release 4.0, using
! 4569: the Python syntax. PCRE now supports both the Perl and the Python syn-
! 4570: tax. Perl allows identically numbered subpatterns to have different
! 4571: names, but PCRE does not.
! 4572:
! 4573: In PCRE, a subpattern can be named in one of three ways: (?<name>...)
! 4574: or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
! 4575: to capturing parentheses from other parts of the pattern, such as back
! 4576: references, recursion, and conditions, can be made by name as well as
! 4577: by number.
! 4578:
! 4579: Names consist of up to 32 alphanumeric characters and underscores.
! 4580: Named capturing parentheses are still allocated numbers as well as
! 4581: names, exactly as if the names were not present. The PCRE API provides
! 4582: function calls for extracting the name-to-number translation table from
! 4583: a compiled pattern. There is also a convenience function for extracting
! 4584: a captured substring by name.
! 4585:
! 4586: By default, a name must be unique within a pattern, but it is possible
! 4587: to relax this constraint by setting the PCRE_DUPNAMES option at compile
! 4588: time. (Duplicate names are also always permitted for subpatterns with
! 4589: the same number, set up as described in the previous section.) Dupli-
! 4590: cate names can be useful for patterns where only one instance of the
! 4591: named parentheses can match. Suppose you want to match the name of a
! 4592: weekday, either as a 3-letter abbreviation or as the full name, and in
! 4593: both cases you want to extract the abbreviation. This pattern (ignoring
! 4594: the line breaks) does the job:
! 4595:
! 4596: (?<DN>Mon|Fri|Sun)(?:day)?|
! 4597: (?<DN>Tue)(?:sday)?|
! 4598: (?<DN>Wed)(?:nesday)?|
! 4599: (?<DN>Thu)(?:rsday)?|
! 4600: (?<DN>Sat)(?:urday)?
! 4601:
! 4602: There are five capturing substrings, but only one is ever set after a
! 4603: match. (An alternative way of solving this problem is to use a "branch
! 4604: reset" subpattern, as described in the previous section.)
! 4605:
! 4606: The convenience function for extracting the data by name returns the
! 4607: substring for the first (and in this example, the only) subpattern of
! 4608: that name that matched. This saves searching to find which numbered
! 4609: subpattern it was.
! 4610:
! 4611: If you make a back reference to a non-unique named subpattern from
! 4612: elsewhere in the pattern, the one that corresponds to the first occur-
! 4613: rence of the name is used. In the absence of duplicate numbers (see the
! 4614: previous section) this is the one with the lowest number. If you use a
! 4615: named reference in a condition test (see the section about conditions
! 4616: below), either to check whether a subpattern has matched, or to check
! 4617: for recursion, all subpatterns with the same name are tested. If the
! 4618: condition is true for any one of them, the overall condition is true.
! 4619: This is the same behaviour as testing by number. For further details of
! 4620: the interfaces for handling named subpatterns, see the pcreapi documen-
! 4621: tation.
! 4622:
! 4623: Warning: You cannot use different names to distinguish between two sub-
! 4624: patterns with the same number because PCRE uses only the numbers when
! 4625: matching. For this reason, an error is given at compile time if differ-
! 4626: ent names are given to subpatterns with the same number. However, you
! 4627: can give the same name to subpatterns with the same number, even when
! 4628: PCRE_DUPNAMES is not set.
! 4629:
! 4630:
! 4631: REPETITION
! 4632:
! 4633: Repetition is specified by quantifiers, which can follow any of the
! 4634: following items:
! 4635:
! 4636: a literal data character
! 4637: the dot metacharacter
! 4638: the \C escape sequence
! 4639: the \X escape sequence (in UTF-8 mode with Unicode properties)
! 4640: the \R escape sequence
! 4641: an escape such as \d or \pL that matches a single character
! 4642: a character class
! 4643: a back reference (see next section)
! 4644: a parenthesized subpattern (including assertions)
! 4645: a subroutine call to a subpattern (recursive or otherwise)
! 4646:
! 4647: The general repetition quantifier specifies a minimum and maximum num-
! 4648: ber of permitted matches, by giving the two numbers in curly brackets
! 4649: (braces), separated by a comma. The numbers must be less than 65536,
! 4650: and the first must be less than or equal to the second. For example:
! 4651:
! 4652: z{2,4}
! 4653:
! 4654: matches "zz", "zzz", or "zzzz". A closing brace on its own is not a
! 4655: special character. If the second number is omitted, but the comma is
! 4656: present, there is no upper limit; if the second number and the comma
! 4657: are both omitted, the quantifier specifies an exact number of required
! 4658: matches. Thus
! 4659:
! 4660: [aeiou]{3,}
! 4661:
! 4662: matches at least 3 successive vowels, but may match many more, while
! 4663:
! 4664: \d{8}
! 4665:
! 4666: matches exactly 8 digits. An opening curly bracket that appears in a
! 4667: position where a quantifier is not allowed, or one that does not match
! 4668: the syntax of a quantifier, is taken as a literal character. For exam-
! 4669: ple, {,6} is not a quantifier, but a literal string of four characters.
! 4670:
! 4671: In UTF-8 mode, quantifiers apply to UTF-8 characters rather than to
! 4672: individual bytes. Thus, for example, \x{100}{2} matches two UTF-8 char-
! 4673: acters, each of which is represented by a two-byte sequence. Similarly,
! 4674: when Unicode property support is available, \X{3} matches three Unicode
! 4675: extended sequences, each of which may be several bytes long (and they
! 4676: may be of different lengths).
! 4677:
! 4678: The quantifier {0} is permitted, causing the expression to behave as if
! 4679: the previous item and the quantifier were not present. This may be use-
! 4680: ful for subpatterns that are referenced as subroutines from elsewhere
! 4681: in the pattern (but see also the section entitled "Defining subpatterns
! 4682: for use by reference only" below). Items other than subpatterns that
! 4683: have a {0} quantifier are omitted from the compiled pattern.
! 4684:
! 4685: For convenience, the three most common quantifiers have single-charac-
! 4686: ter abbreviations:
! 4687:
! 4688: * is equivalent to {0,}
! 4689: + is equivalent to {1,}
! 4690: ? is equivalent to {0,1}
! 4691:
! 4692: It is possible to construct infinite loops by following a subpattern
! 4693: that can match no characters with a quantifier that has no upper limit,
! 4694: for example:
! 4695:
! 4696: (a?)*
! 4697:
! 4698: Earlier versions of Perl and PCRE used to give an error at compile time
! 4699: for such patterns. However, because there are cases where this can be
! 4700: useful, such patterns are now accepted, but if any repetition of the
! 4701: subpattern does in fact match no characters, the loop is forcibly bro-
! 4702: ken.
! 4703:
! 4704: By default, the quantifiers are "greedy", that is, they match as much
! 4705: as possible (up to the maximum number of permitted times), without
! 4706: causing the rest of the pattern to fail. The classic example of where
! 4707: this gives problems is in trying to match comments in C programs. These
! 4708: appear between /* and */ and within the comment, individual * and /
! 4709: characters may appear. An attempt to match C comments by applying the
! 4710: pattern
! 4711:
! 4712: /\*.*\*/
! 4713:
! 4714: to the string
! 4715:
! 4716: /* first comment */ not comment /* second comment */
! 4717:
! 4718: fails, because it matches the entire string owing to the greediness of
! 4719: the .* item.
! 4720:
! 4721: However, if a quantifier is followed by a question mark, it ceases to
! 4722: be greedy, and instead matches the minimum number of times possible, so
! 4723: the pattern
! 4724:
! 4725: /\*.*?\*/
! 4726:
! 4727: does the right thing with the C comments. The meaning of the various
! 4728: quantifiers is not otherwise changed, just the preferred number of
! 4729: matches. Do not confuse this use of question mark with its use as a
! 4730: quantifier in its own right. Because it has two uses, it can sometimes
! 4731: appear doubled, as in
! 4732:
! 4733: \d??\d
! 4734:
! 4735: which matches one digit by preference, but can match two if that is the
! 4736: only way the rest of the pattern matches.
! 4737:
! 4738: If the PCRE_UNGREEDY option is set (an option that is not available in
! 4739: Perl), the quantifiers are not greedy by default, but individual ones
! 4740: can be made greedy by following them with a question mark. In other
! 4741: words, it inverts the default behaviour.
! 4742:
! 4743: When a parenthesized subpattern is quantified with a minimum repeat
! 4744: count that is greater than 1 or with a limited maximum, more memory is
! 4745: required for the compiled pattern, in proportion to the size of the
! 4746: minimum or maximum.
! 4747:
! 4748: If a pattern starts with .* or .{0,} and the PCRE_DOTALL option (equiv-
! 4749: alent to Perl's /s) is set, thus allowing the dot to match newlines,
! 4750: the pattern is implicitly anchored, because whatever follows will be
! 4751: tried against every character position in the subject string, so there
! 4752: is no point in retrying the overall match at any position after the
! 4753: first. PCRE normally treats such a pattern as though it were preceded
! 4754: by \A.
! 4755:
! 4756: In cases where it is known that the subject string contains no new-
! 4757: lines, it is worth setting PCRE_DOTALL in order to obtain this opti-
! 4758: mization, or alternatively using ^ to indicate anchoring explicitly.
! 4759:
! 4760: However, there is one situation where the optimization cannot be used.
! 4761: When .* is inside capturing parentheses that are the subject of a back
! 4762: reference elsewhere in the pattern, a match at the start may fail where
! 4763: a later one succeeds. Consider, for example:
! 4764:
! 4765: (.*)abc\1
! 4766:
! 4767: If the subject is "xyz123abc123" the match point is the fourth charac-
! 4768: ter. For this reason, such a pattern is not implicitly anchored.
! 4769:
! 4770: When a capturing subpattern is repeated, the value captured is the sub-
! 4771: string that matched the final iteration. For example, after
! 4772:
! 4773: (tweedle[dume]{3}\s*)+
! 4774:
! 4775: has matched "tweedledum tweedledee" the value of the captured substring
! 4776: is "tweedledee". However, if there are nested capturing subpatterns,
! 4777: the corresponding captured values may have been set in previous itera-
! 4778: tions. For example, after
! 4779:
! 4780: /(a|(b))+/
! 4781:
! 4782: matches "aba" the value of the second captured substring is "b".
! 4783:
! 4784:
! 4785: ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS
! 4786:
! 4787: With both maximizing ("greedy") and minimizing ("ungreedy" or "lazy")
! 4788: repetition, failure of what follows normally causes the repeated item
! 4789: to be re-evaluated to see if a different number of repeats allows the
! 4790: rest of the pattern to match. Sometimes it is useful to prevent this,
! 4791: either to change the nature of the match, or to cause it fail earlier
! 4792: than it otherwise might, when the author of the pattern knows there is
! 4793: no point in carrying on.
! 4794:
! 4795: Consider, for example, the pattern \d+foo when applied to the subject
! 4796: line
! 4797:
! 4798: 123456bar
! 4799:
! 4800: After matching all 6 digits and then failing to match "foo", the normal
! 4801: action of the matcher is to try again with only 5 digits matching the
! 4802: \d+ item, and then with 4, and so on, before ultimately failing.
! 4803: "Atomic grouping" (a term taken from Jeffrey Friedl's book) provides
! 4804: the means for specifying that once a subpattern has matched, it is not
! 4805: to be re-evaluated in this way.
! 4806:
! 4807: If we use atomic grouping for the previous example, the matcher gives
! 4808: up immediately on failing to match "foo" the first time. The notation
! 4809: is a kind of special parenthesis, starting with (?> as in this example:
! 4810:
! 4811: (?>\d+)foo
! 4812:
! 4813: This kind of parenthesis "locks up" the part of the pattern it con-
! 4814: tains once it has matched, and a failure further into the pattern is
! 4815: prevented from backtracking into it. Backtracking past it to previous
! 4816: items, however, works as normal.
! 4817:
! 4818: An alternative description is that a subpattern of this type matches
! 4819: the string of characters that an identical standalone pattern would
! 4820: match, if anchored at the current point in the subject string.
! 4821:
! 4822: Atomic grouping subpatterns are not capturing subpatterns. Simple cases
! 4823: such as the above example can be thought of as a maximizing repeat that
! 4824: must swallow everything it can. So, while both \d+ and \d+? are pre-
! 4825: pared to adjust the number of digits they match in order to make the
! 4826: rest of the pattern match, (?>\d+) can only match an entire sequence of
! 4827: digits.
! 4828:
! 4829: Atomic groups in general can of course contain arbitrarily complicated
! 4830: subpatterns, and can be nested. However, when the subpattern for an
! 4831: atomic group is just a single repeated item, as in the example above, a
! 4832: simpler notation, called a "possessive quantifier" can be used. This
! 4833: consists of an additional + character following a quantifier. Using
! 4834: this notation, the previous example can be rewritten as
! 4835:
! 4836: \d++foo
! 4837:
! 4838: Note that a possessive quantifier can be used with an entire group, for
! 4839: example:
! 4840:
! 4841: (abc|xyz){2,3}+
! 4842:
! 4843: Possessive quantifiers are always greedy; the setting of the
! 4844: PCRE_UNGREEDY option is ignored. They are a convenient notation for the
! 4845: simpler forms of atomic group. However, there is no difference in the
! 4846: meaning of a possessive quantifier and the equivalent atomic group,
! 4847: though there may be a performance difference; possessive quantifiers
! 4848: should be slightly faster.
! 4849:
! 4850: The possessive quantifier syntax is an extension to the Perl 5.8 syn-
! 4851: tax. Jeffrey Friedl originated the idea (and the name) in the first
! 4852: edition of his book. Mike McCloskey liked it, so implemented it when he
! 4853: built Sun's Java package, and PCRE copied it from there. It ultimately
! 4854: found its way into Perl at release 5.10.
! 4855:
! 4856: PCRE has an optimization that automatically "possessifies" certain sim-
! 4857: ple pattern constructs. For example, the sequence A+B is treated as
! 4858: A++B because there is no point in backtracking into a sequence of A's
! 4859: when B must follow.
! 4860:
! 4861: When a pattern contains an unlimited repeat inside a subpattern that
! 4862: can itself be repeated an unlimited number of times, the use of an
! 4863: atomic group is the only way to avoid some failing matches taking a
! 4864: very long time indeed. The pattern
! 4865:
! 4866: (\D+|<\d+>)*[!?]
! 4867:
! 4868: matches an unlimited number of substrings that either consist of non-
! 4869: digits, or digits enclosed in <>, followed by either ! or ?. When it
! 4870: matches, it runs quickly. However, if it is applied to
! 4871:
! 4872: aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
! 4873:
! 4874: it takes a long time before reporting failure. This is because the
! 4875: string can be divided between the internal \D+ repeat and the external
! 4876: * repeat in a large number of ways, and all have to be tried. (The
! 4877: example uses [!?] rather than a single character at the end, because
! 4878: both PCRE and Perl have an optimization that allows for fast failure
! 4879: when a single character is used. They remember the last single charac-
! 4880: ter that is required for a match, and fail early if it is not present
! 4881: in the string.) If the pattern is changed so that it uses an atomic
! 4882: group, like this:
! 4883:
! 4884: ((?>\D+)|<\d+>)*[!?]
! 4885:
! 4886: sequences of non-digits cannot be broken, and failure happens quickly.
! 4887:
! 4888:
! 4889: BACK REFERENCES
! 4890:
! 4891: Outside a character class, a backslash followed by a digit greater than
! 4892: 0 (and possibly further digits) is a back reference to a capturing sub-
! 4893: pattern earlier (that is, to its left) in the pattern, provided there
! 4894: have been that many previous capturing left parentheses.
! 4895:
! 4896: However, if the decimal number following the backslash is less than 10,
! 4897: it is always taken as a back reference, and causes an error only if
! 4898: there are not that many capturing left parentheses in the entire pat-
! 4899: tern. In other words, the parentheses that are referenced need not be
! 4900: to the left of the reference for numbers less than 10. A "forward back
! 4901: reference" of this type can make sense when a repetition is involved
! 4902: and the subpattern to the right has participated in an earlier itera-
! 4903: tion.
! 4904:
! 4905: It is not possible to have a numerical "forward back reference" to a
! 4906: subpattern whose number is 10 or more using this syntax because a
! 4907: sequence such as \50 is interpreted as a character defined in octal.
! 4908: See the subsection entitled "Non-printing characters" above for further
! 4909: details of the handling of digits following a backslash. There is no
! 4910: such problem when named parentheses are used. A back reference to any
! 4911: subpattern is possible using named parentheses (see below).
! 4912:
! 4913: Another way of avoiding the ambiguity inherent in the use of digits
! 4914: following a backslash is to use the \g escape sequence. This escape
! 4915: must be followed by an unsigned number or a negative number, optionally
! 4916: enclosed in braces. These examples are all identical:
! 4917:
! 4918: (ring), \1
! 4919: (ring), \g1
! 4920: (ring), \g{1}
! 4921:
! 4922: An unsigned number specifies an absolute reference without the ambigu-
! 4923: ity that is present in the older syntax. It is also useful when literal
! 4924: digits follow the reference. A negative number is a relative reference.
! 4925: Consider this example:
! 4926:
! 4927: (abc(def)ghi)\g{-1}
! 4928:
! 4929: The sequence \g{-1} is a reference to the most recently started captur-
! 4930: ing subpattern before \g, that is, is it equivalent to \2 in this exam-
! 4931: ple. Similarly, \g{-2} would be equivalent to \1. The use of relative
! 4932: references can be helpful in long patterns, and also in patterns that
! 4933: are created by joining together fragments that contain references
! 4934: within themselves.
! 4935:
! 4936: A back reference matches whatever actually matched the capturing sub-
! 4937: pattern in the current subject string, rather than anything matching
! 4938: the subpattern itself (see "Subpatterns as subroutines" below for a way
! 4939: of doing that). So the pattern
! 4940:
! 4941: (sens|respons)e and \1ibility
! 4942:
! 4943: matches "sense and sensibility" and "response and responsibility", but
! 4944: not "sense and responsibility". If caseful matching is in force at the
! 4945: time of the back reference, the case of letters is relevant. For exam-
! 4946: ple,
! 4947:
! 4948: ((?i)rah)\s+\1
! 4949:
! 4950: matches "rah rah" and "RAH RAH", but not "RAH rah", even though the
! 4951: original capturing subpattern is matched caselessly.
! 4952:
! 4953: There are several different ways of writing back references to named
! 4954: subpatterns. The .NET syntax \k{name} and the Perl syntax \k<name> or
! 4955: \k'name' are supported, as is the Python syntax (?P=name). Perl 5.10's
! 4956: unified back reference syntax, in which \g can be used for both numeric
! 4957: and named references, is also supported. We could rewrite the above
! 4958: example in any of the following ways:
! 4959:
! 4960: (?<p1>(?i)rah)\s+\k<p1>
! 4961: (?'p1'(?i)rah)\s+\k{p1}
! 4962: (?P<p1>(?i)rah)\s+(?P=p1)
! 4963: (?<p1>(?i)rah)\s+\g{p1}
! 4964:
! 4965: A subpattern that is referenced by name may appear in the pattern
! 4966: before or after the reference.
! 4967:
! 4968: There may be more than one back reference to the same subpattern. If a
! 4969: subpattern has not actually been used in a particular match, any back
! 4970: references to it always fail by default. For example, the pattern
! 4971:
! 4972: (a|(bc))\2
! 4973:
! 4974: always fails if it starts to match "a" rather than "bc". However, if
! 4975: the PCRE_JAVASCRIPT_COMPAT option is set at compile time, a back refer-
! 4976: ence to an unset value matches an empty string.
! 4977:
! 4978: Because there may be many capturing parentheses in a pattern, all dig-
! 4979: its following a backslash are taken as part of a potential back refer-
! 4980: ence number. If the pattern continues with a digit character, some
! 4981: delimiter must be used to terminate the back reference. If the
! 4982: PCRE_EXTENDED option is set, this can be whitespace. Otherwise, the \g{
! 4983: syntax or an empty comment (see "Comments" below) can be used.
! 4984:
! 4985: Recursive back references
! 4986:
! 4987: A back reference that occurs inside the parentheses to which it refers
! 4988: fails when the subpattern is first used, so, for example, (a\1) never
! 4989: matches. However, such references can be useful inside repeated sub-
! 4990: patterns. For example, the pattern
! 4991:
! 4992: (a|b\1)+
! 4993:
! 4994: matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
! 4995: ation of the subpattern, the back reference matches the character
! 4996: string corresponding to the previous iteration. In order for this to
! 4997: work, the pattern must be such that the first iteration does not need
! 4998: to match the back reference. This can be done using alternation, as in
! 4999: the example above, or by a quantifier with a minimum of zero.
! 5000:
! 5001: Back references of this type cause the group that they reference to be
! 5002: treated as an atomic group. Once the whole group has been matched, a
! 5003: subsequent matching failure cannot cause backtracking into the middle
! 5004: of the group.
! 5005:
! 5006:
! 5007: ASSERTIONS
! 5008:
! 5009: An assertion is a test on the characters following or preceding the
! 5010: current matching point that does not actually consume any characters.
! 5011: The simple assertions coded as \b, \B, \A, \G, \Z, \z, ^ and $ are
! 5012: described above.
! 5013:
! 5014: More complicated assertions are coded as subpatterns. There are two
! 5015: kinds: those that look ahead of the current position in the subject
! 5016: string, and those that look behind it. An assertion subpattern is
! 5017: matched in the normal way, except that it does not cause the current
! 5018: matching position to be changed.
! 5019:
! 5020: Assertion subpatterns are not capturing subpatterns. If such an asser-
! 5021: tion contains capturing subpatterns within it, these are counted for
! 5022: the purposes of numbering the capturing subpatterns in the whole pat-
! 5023: tern. However, substring capturing is carried out only for positive
! 5024: assertions, because it does not make sense for negative assertions.
! 5025:
! 5026: For compatibility with Perl, assertion subpatterns may be repeated;
! 5027: though it makes no sense to assert the same thing several times, the
! 5028: side effect of capturing parentheses may occasionally be useful. In
! 5029: practice, there only three cases:
! 5030:
! 5031: (1) If the quantifier is {0}, the assertion is never obeyed during
! 5032: matching. However, it may contain internal capturing parenthesized
! 5033: groups that are called from elsewhere via the subroutine mechanism.
! 5034:
! 5035: (2) If quantifier is {0,n} where n is greater than zero, it is treated
! 5036: as if it were {0,1}. At run time, the rest of the pattern match is
! 5037: tried with and without the assertion, the order depending on the greed-
! 5038: iness of the quantifier.
! 5039:
! 5040: (3) If the minimum repetition is greater than zero, the quantifier is
! 5041: ignored. The assertion is obeyed just once when encountered during
! 5042: matching.
! 5043:
! 5044: Lookahead assertions
! 5045:
! 5046: Lookahead assertions start with (?= for positive assertions and (?! for
! 5047: negative assertions. For example,
! 5048:
! 5049: \w+(?=;)
! 5050:
! 5051: matches a word followed by a semicolon, but does not include the semi-
! 5052: colon in the match, and
! 5053:
! 5054: foo(?!bar)
! 5055:
! 5056: matches any occurrence of "foo" that is not followed by "bar". Note
! 5057: that the apparently similar pattern
! 5058:
! 5059: (?!foo)bar
! 5060:
! 5061: does not find an occurrence of "bar" that is preceded by something
! 5062: other than "foo"; it finds any occurrence of "bar" whatsoever, because
! 5063: the assertion (?!foo) is always true when the next three characters are
! 5064: "bar". A lookbehind assertion is needed to achieve the other effect.
! 5065:
! 5066: If you want to force a matching failure at some point in a pattern, the
! 5067: most convenient way to do it is with (?!) because an empty string
! 5068: always matches, so an assertion that requires there not to be an empty
! 5069: string must always fail. The backtracking control verb (*FAIL) or (*F)
! 5070: is a synonym for (?!).
! 5071:
! 5072: Lookbehind assertions
! 5073:
! 5074: Lookbehind assertions start with (?<= for positive assertions and (?<!
! 5075: for negative assertions. For example,
! 5076:
! 5077: (?<!foo)bar
! 5078:
! 5079: does find an occurrence of "bar" that is not preceded by "foo". The
! 5080: contents of a lookbehind assertion are restricted such that all the
! 5081: strings it matches must have a fixed length. However, if there are sev-
! 5082: eral top-level alternatives, they do not all have to have the same
! 5083: fixed length. Thus
! 5084:
! 5085: (?<=bullock|donkey)
! 5086:
! 5087: is permitted, but
! 5088:
! 5089: (?<!dogs?|cats?)
! 5090:
! 5091: causes an error at compile time. Branches that match different length
! 5092: strings are permitted only at the top level of a lookbehind assertion.
! 5093: This is an extension compared with Perl, which requires all branches to
! 5094: match the same length of string. An assertion such as
! 5095:
! 5096: (?<=ab(c|de))
! 5097:
! 5098: is not permitted, because its single top-level branch can match two
! 5099: different lengths, but it is acceptable to PCRE if rewritten to use two
! 5100: top-level branches:
! 5101:
! 5102: (?<=abc|abde)
! 5103:
! 5104: In some cases, the escape sequence \K (see above) can be used instead
! 5105: of a lookbehind assertion to get round the fixed-length restriction.
! 5106:
! 5107: The implementation of lookbehind assertions is, for each alternative,
! 5108: to temporarily move the current position back by the fixed length and
! 5109: then try to match. If there are insufficient characters before the cur-
! 5110: rent position, the assertion fails.
! 5111:
! 5112: In UTF-8 mode, PCRE does not allow the \C escape (which matches a sin-
! 5113: gle byte, even in UTF-8 mode) to appear in lookbehind assertions,
! 5114: because it makes it impossible to calculate the length of the lookbe-
! 5115: hind. The \X and \R escapes, which can match different numbers of
! 5116: bytes, are also not permitted.
! 5117:
! 5118: "Subroutine" calls (see below) such as (?2) or (?&X) are permitted in
! 5119: lookbehinds, as long as the subpattern matches a fixed-length string.
! 5120: Recursion, however, is not supported.
! 5121:
! 5122: Possessive quantifiers can be used in conjunction with lookbehind
! 5123: assertions to specify efficient matching of fixed-length strings at the
! 5124: end of subject strings. Consider a simple pattern such as
! 5125:
! 5126: abcd$
! 5127:
! 5128: when applied to a long string that does not match. Because matching
! 5129: proceeds from left to right, PCRE will look for each "a" in the subject
! 5130: and then see if what follows matches the rest of the pattern. If the
! 5131: pattern is specified as
! 5132:
! 5133: ^.*abcd$
! 5134:
! 5135: the initial .* matches the entire string at first, but when this fails
! 5136: (because there is no following "a"), it backtracks to match all but the
! 5137: last character, then all but the last two characters, and so on. Once
! 5138: again the search for "a" covers the entire string, from right to left,
! 5139: so we are no better off. However, if the pattern is written as
! 5140:
! 5141: ^.*+(?<=abcd)
! 5142:
! 5143: there can be no backtracking for the .*+ item; it can match only the
! 5144: entire string. The subsequent lookbehind assertion does a single test
! 5145: on the last four characters. If it fails, the match fails immediately.
! 5146: For long strings, this approach makes a significant difference to the
! 5147: processing time.
! 5148:
! 5149: Using multiple assertions
! 5150:
! 5151: Several assertions (of any sort) may occur in succession. For example,
! 5152:
! 5153: (?<=\d{3})(?<!999)foo
! 5154:
! 5155: matches "foo" preceded by three digits that are not "999". Notice that
! 5156: each of the assertions is applied independently at the same point in
! 5157: the subject string. First there is a check that the previous three
! 5158: characters are all digits, and then there is a check that the same
! 5159: three characters are not "999". This pattern does not match "foo" pre-
! 5160: ceded by six characters, the first of which are digits and the last
! 5161: three of which are not "999". For example, it doesn't match "123abc-
! 5162: foo". A pattern to do that is
! 5163:
! 5164: (?<=\d{3}...)(?<!999)foo
! 5165:
! 5166: This time the first assertion looks at the preceding six characters,
! 5167: checking that the first three are digits, and then the second assertion
! 5168: checks that the preceding three characters are not "999".
! 5169:
! 5170: Assertions can be nested in any combination. For example,
! 5171:
! 5172: (?<=(?<!foo)bar)baz
! 5173:
! 5174: matches an occurrence of "baz" that is preceded by "bar" which in turn
! 5175: is not preceded by "foo", while
! 5176:
! 5177: (?<=\d{3}(?!999)...)foo
! 5178:
! 5179: is another pattern that matches "foo" preceded by three digits and any
! 5180: three characters that are not "999".
! 5181:
! 5182:
! 5183: CONDITIONAL SUBPATTERNS
! 5184:
! 5185: It is possible to cause the matching process to obey a subpattern con-
! 5186: ditionally or to choose between two alternative subpatterns, depending
! 5187: on the result of an assertion, or whether a specific capturing subpat-
! 5188: tern has already been matched. The two possible forms of conditional
! 5189: subpattern are:
! 5190:
! 5191: (?(condition)yes-pattern)
! 5192: (?(condition)yes-pattern|no-pattern)
! 5193:
! 5194: If the condition is satisfied, the yes-pattern is used; otherwise the
! 5195: no-pattern (if present) is used. If there are more than two alterna-
! 5196: tives in the subpattern, a compile-time error occurs. Each of the two
! 5197: alternatives may itself contain nested subpatterns of any form, includ-
! 5198: ing conditional subpatterns; the restriction to two alternatives
! 5199: applies only at the level of the condition. This pattern fragment is an
! 5200: example where the alternatives are complex:
! 5201:
! 5202: (?(1) (A|B|C) | (D | (?(2)E|F) | E) )
! 5203:
! 5204:
! 5205: There are four kinds of condition: references to subpatterns, refer-
! 5206: ences to recursion, a pseudo-condition called DEFINE, and assertions.
! 5207:
! 5208: Checking for a used subpattern by number
! 5209:
! 5210: If the text between the parentheses consists of a sequence of digits,
! 5211: the condition is true if a capturing subpattern of that number has pre-
! 5212: viously matched. If there is more than one capturing subpattern with
! 5213: the same number (see the earlier section about duplicate subpattern
! 5214: numbers), the condition is true if any of them have matched. An alter-
! 5215: native notation is to precede the digits with a plus or minus sign. In
! 5216: this case, the subpattern number is relative rather than absolute. The
! 5217: most recently opened parentheses can be referenced by (?(-1), the next
! 5218: most recent by (?(-2), and so on. Inside loops it can also make sense
! 5219: to refer to subsequent groups. The next parentheses to be opened can be
! 5220: referenced as (?(+1), and so on. (The value zero in any of these forms
! 5221: is not used; it provokes a compile-time error.)
! 5222:
! 5223: Consider the following pattern, which contains non-significant white
! 5224: space to make it more readable (assume the PCRE_EXTENDED option) and to
! 5225: divide it into three parts for ease of discussion:
! 5226:
! 5227: ( \( )? [^()]+ (?(1) \) )
! 5228:
! 5229: The first part matches an optional opening parenthesis, and if that
! 5230: character is present, sets it as the first captured substring. The sec-
! 5231: ond part matches one or more characters that are not parentheses. The
! 5232: third part is a conditional subpattern that tests whether or not the
! 5233: first set of parentheses matched. If they did, that is, if subject
! 5234: started with an opening parenthesis, the condition is true, and so the
! 5235: yes-pattern is executed and a closing parenthesis is required. Other-
! 5236: wise, since no-pattern is not present, the subpattern matches nothing.
! 5237: In other words, this pattern matches a sequence of non-parentheses,
! 5238: optionally enclosed in parentheses.
! 5239:
! 5240: If you were embedding this pattern in a larger one, you could use a
! 5241: relative reference:
! 5242:
! 5243: ...other stuff... ( \( )? [^()]+ (?(-1) \) ) ...
! 5244:
! 5245: This makes the fragment independent of the parentheses in the larger
! 5246: pattern.
! 5247:
! 5248: Checking for a used subpattern by name
! 5249:
! 5250: Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
! 5251: used subpattern by name. For compatibility with earlier versions of
! 5252: PCRE, which had this facility before Perl, the syntax (?(name)...) is
! 5253: also recognized. However, there is a possible ambiguity with this syn-
! 5254: tax, because subpattern names may consist entirely of digits. PCRE
! 5255: looks first for a named subpattern; if it cannot find one and the name
! 5256: consists entirely of digits, PCRE looks for a subpattern of that num-
! 5257: ber, which must be greater than zero. Using subpattern names that con-
! 5258: sist entirely of digits is not recommended.
! 5259:
! 5260: Rewriting the above example to use a named subpattern gives this:
! 5261:
! 5262: (?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
! 5263:
! 5264: If the name used in a condition of this kind is a duplicate, the test
! 5265: is applied to all subpatterns of the same name, and is true if any one
! 5266: of them has matched.
! 5267:
! 5268: Checking for pattern recursion
! 5269:
! 5270: If the condition is the string (R), and there is no subpattern with the
! 5271: name R, the condition is true if a recursive call to the whole pattern
! 5272: or any subpattern has been made. If digits or a name preceded by amper-
! 5273: sand follow the letter R, for example:
! 5274:
! 5275: (?(R3)...) or (?(R&name)...)
! 5276:
! 5277: the condition is true if the most recent recursion is into a subpattern
! 5278: whose number or name is given. This condition does not check the entire
! 5279: recursion stack. If the name used in a condition of this kind is a
! 5280: duplicate, the test is applied to all subpatterns of the same name, and
! 5281: is true if any one of them is the most recent recursion.
! 5282:
! 5283: At "top level", all these recursion test conditions are false. The
! 5284: syntax for recursive patterns is described below.
! 5285:
! 5286: Defining subpatterns for use by reference only
! 5287:
! 5288: If the condition is the string (DEFINE), and there is no subpattern
! 5289: with the name DEFINE, the condition is always false. In this case,
! 5290: there may be only one alternative in the subpattern. It is always
! 5291: skipped if control reaches this point in the pattern; the idea of
! 5292: DEFINE is that it can be used to define subroutines that can be refer-
! 5293: enced from elsewhere. (The use of subroutines is described below.) For
! 5294: example, a pattern to match an IPv4 address such as "192.168.23.245"
! 5295: could be written like this (ignore whitespace and line breaks):
! 5296:
! 5297: (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
! 5298: \b (?&byte) (\.(?&byte)){3} \b
! 5299:
! 5300: The first part of the pattern is a DEFINE group inside which a another
! 5301: group named "byte" is defined. This matches an individual component of
! 5302: an IPv4 address (a number less than 256). When matching takes place,
! 5303: this part of the pattern is skipped because DEFINE acts like a false
! 5304: condition. The rest of the pattern uses references to the named group
! 5305: to match the four dot-separated components of an IPv4 address, insist-
! 5306: ing on a word boundary at each end.
! 5307:
! 5308: Assertion conditions
! 5309:
! 5310: If the condition is not in any of the above formats, it must be an
! 5311: assertion. This may be a positive or negative lookahead or lookbehind
! 5312: assertion. Consider this pattern, again containing non-significant
! 5313: white space, and with the two alternatives on the second line:
! 5314:
! 5315: (?(?=[^a-z]*[a-z])
! 5316: \d{2}-[a-z]{3}-\d{2} | \d{2}-\d{2}-\d{2} )
! 5317:
! 5318: The condition is a positive lookahead assertion that matches an
! 5319: optional sequence of non-letters followed by a letter. In other words,
! 5320: it tests for the presence of at least one letter in the subject. If a
! 5321: letter is found, the subject is matched against the first alternative;
! 5322: otherwise it is matched against the second. This pattern matches
! 5323: strings in one of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
! 5324: letters and dd are digits.
! 5325:
! 5326:
! 5327: COMMENTS
! 5328:
! 5329: There are two ways of including comments in patterns that are processed
! 5330: by PCRE. In both cases, the start of the comment must not be in a char-
! 5331: acter class, nor in the middle of any other sequence of related charac-
! 5332: ters such as (?: or a subpattern name or number. The characters that
! 5333: make up a comment play no part in the pattern matching.
! 5334:
! 5335: The sequence (?# marks the start of a comment that continues up to the
! 5336: next closing parenthesis. Nested parentheses are not permitted. If the
! 5337: PCRE_EXTENDED option is set, an unescaped # character also introduces a
! 5338: comment, which in this case continues to immediately after the next
! 5339: newline character or character sequence in the pattern. Which charac-
! 5340: ters are interpreted as newlines is controlled by the options passed to
! 5341: pcre_compile() or by a special sequence at the start of the pattern, as
! 5342: described in the section entitled "Newline conventions" above. Note
! 5343: that the end of this type of comment is a literal newline sequence in
! 5344: the pattern; escape sequences that happen to represent a newline do not
! 5345: count. For example, consider this pattern when PCRE_EXTENDED is set,
! 5346: and the default newline convention is in force:
! 5347:
! 5348: abc #comment \n still comment
! 5349:
! 5350: On encountering the # character, pcre_compile() skips along, looking
! 5351: for a newline in the pattern. The sequence \n is still literal at this
! 5352: stage, so it does not terminate the comment. Only an actual character
! 5353: with the code value 0x0a (the default newline) does so.
! 5354:
! 5355:
! 5356: RECURSIVE PATTERNS
! 5357:
! 5358: Consider the problem of matching a string in parentheses, allowing for
! 5359: unlimited nested parentheses. Without the use of recursion, the best
! 5360: that can be done is to use a pattern that matches up to some fixed
! 5361: depth of nesting. It is not possible to handle an arbitrary nesting
! 5362: depth.
! 5363:
! 5364: For some time, Perl has provided a facility that allows regular expres-
! 5365: sions to recurse (amongst other things). It does this by interpolating
! 5366: Perl code in the expression at run time, and the code can refer to the
! 5367: expression itself. A Perl pattern using code interpolation to solve the
! 5368: parentheses problem can be created like this:
! 5369:
! 5370: $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;
! 5371:
! 5372: The (?p{...}) item interpolates Perl code at run time, and in this case
! 5373: refers recursively to the pattern in which it appears.
! 5374:
! 5375: Obviously, PCRE cannot support the interpolation of Perl code. Instead,
! 5376: it supports special syntax for recursion of the entire pattern, and
! 5377: also for individual subpattern recursion. After its introduction in
! 5378: PCRE and Python, this kind of recursion was subsequently introduced
! 5379: into Perl at release 5.10.
! 5380:
! 5381: A special item that consists of (? followed by a number greater than
! 5382: zero and a closing parenthesis is a recursive subroutine call of the
! 5383: subpattern of the given number, provided that it occurs inside that
! 5384: subpattern. (If not, it is a non-recursive subroutine call, which is
! 5385: described in the next section.) The special item (?R) or (?0) is a
! 5386: recursive call of the entire regular expression.
! 5387:
! 5388: This PCRE pattern solves the nested parentheses problem (assume the
! 5389: PCRE_EXTENDED option is set so that white space is ignored):
! 5390:
! 5391: \( ( [^()]++ | (?R) )* \)
! 5392:
! 5393: First it matches an opening parenthesis. Then it matches any number of
! 5394: substrings which can either be a sequence of non-parentheses, or a
! 5395: recursive match of the pattern itself (that is, a correctly parenthe-
! 5396: sized substring). Finally there is a closing parenthesis. Note the use
! 5397: of a possessive quantifier to avoid backtracking into sequences of non-
! 5398: parentheses.
! 5399:
! 5400: If this were part of a larger pattern, you would not want to recurse
! 5401: the entire pattern, so instead you could use this:
! 5402:
! 5403: ( \( ( [^()]++ | (?1) )* \) )
! 5404:
! 5405: We have put the pattern into parentheses, and caused the recursion to
! 5406: refer to them instead of the whole pattern.
! 5407:
! 5408: In a larger pattern, keeping track of parenthesis numbers can be
! 5409: tricky. This is made easier by the use of relative references. Instead
! 5410: of (?1) in the pattern above you can write (?-2) to refer to the second
! 5411: most recently opened parentheses preceding the recursion. In other
! 5412: words, a negative number counts capturing parentheses leftwards from
! 5413: the point at which it is encountered.
! 5414:
! 5415: It is also possible to refer to subsequently opened parentheses, by
! 5416: writing references such as (?+2). However, these cannot be recursive
! 5417: because the reference is not inside the parentheses that are refer-
! 5418: enced. They are always non-recursive subroutine calls, as described in
! 5419: the next section.
! 5420:
! 5421: An alternative approach is to use named parentheses instead. The Perl
! 5422: syntax for this is (?&name); PCRE's earlier syntax (?P>name) is also
! 5423: supported. We could rewrite the above example as follows:
! 5424:
! 5425: (?<pn> \( ( [^()]++ | (?&pn) )* \) )
! 5426:
! 5427: If there is more than one subpattern with the same name, the earliest
! 5428: one is used.
! 5429:
! 5430: This particular example pattern that we have been looking at contains
! 5431: nested unlimited repeats, and so the use of a possessive quantifier for
! 5432: matching strings of non-parentheses is important when applying the pat-
! 5433: tern to strings that do not match. For example, when this pattern is
! 5434: applied to
! 5435:
! 5436: (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()
! 5437:
! 5438: it yields "no match" quickly. However, if a possessive quantifier is
! 5439: not used, the match runs for a very long time indeed because there are
! 5440: so many different ways the + and * repeats can carve up the subject,
! 5441: and all have to be tested before failure can be reported.
! 5442:
! 5443: At the end of a match, the values of capturing parentheses are those
! 5444: from the outermost level. If you want to obtain intermediate values, a
! 5445: callout function can be used (see below and the pcrecallout documenta-
! 5446: tion). If the pattern above is matched against
! 5447:
! 5448: (ab(cd)ef)
! 5449:
! 5450: the value for the inner capturing parentheses (numbered 2) is "ef",
! 5451: which is the last value taken on at the top level. If a capturing sub-
! 5452: pattern is not matched at the top level, its final captured value is
! 5453: unset, even if it was (temporarily) set at a deeper level during the
! 5454: matching process.
! 5455:
! 5456: If there are more than 15 capturing parentheses in a pattern, PCRE has
! 5457: to obtain extra memory to store data during a recursion, which it does
! 5458: by using pcre_malloc, freeing it via pcre_free afterwards. If no memory
! 5459: can be obtained, the match fails with the PCRE_ERROR_NOMEMORY error.
! 5460:
! 5461: Do not confuse the (?R) item with the condition (R), which tests for
! 5462: recursion. Consider this pattern, which matches text in angle brack-
! 5463: ets, allowing for arbitrary nesting. Only digits are allowed in nested
! 5464: brackets (that is, when recursing), whereas any characters are permit-
! 5465: ted at the outer level.
! 5466:
! 5467: < (?: (?(R) \d++ | [^<>]*+) | (?R)) * >
! 5468:
! 5469: In this pattern, (?(R) is the start of a conditional subpattern, with
! 5470: two different alternatives for the recursive and non-recursive cases.
! 5471: The (?R) item is the actual recursive call.
! 5472:
! 5473: Differences in recursion processing between PCRE and Perl
! 5474:
! 5475: Recursion processing in PCRE differs from Perl in two important ways.
! 5476: In PCRE (like Python, but unlike Perl), a recursive subpattern call is
! 5477: always treated as an atomic group. That is, once it has matched some of
! 5478: the subject string, it is never re-entered, even if it contains untried
! 5479: alternatives and there is a subsequent matching failure. This can be
! 5480: illustrated by the following pattern, which purports to match a palin-
! 5481: dromic string that contains an odd number of characters (for example,
! 5482: "a", "aba", "abcba", "abcdcba"):
! 5483:
! 5484: ^(.|(.)(?1)\2)$
! 5485:
! 5486: The idea is that it either matches a single character, or two identical
! 5487: characters surrounding a sub-palindrome. In Perl, this pattern works;
! 5488: in PCRE it does not if the pattern is longer than three characters.
! 5489: Consider the subject string "abcba":
! 5490:
! 5491: At the top level, the first character is matched, but as it is not at
! 5492: the end of the string, the first alternative fails; the second alterna-
! 5493: tive is taken and the recursion kicks in. The recursive call to subpat-
! 5494: tern 1 successfully matches the next character ("b"). (Note that the
! 5495: beginning and end of line tests are not part of the recursion).
! 5496:
! 5497: Back at the top level, the next character ("c") is compared with what
! 5498: subpattern 2 matched, which was "a". This fails. Because the recursion
! 5499: is treated as an atomic group, there are now no backtracking points,
! 5500: and so the entire match fails. (Perl is able, at this point, to re-
! 5501: enter the recursion and try the second alternative.) However, if the
! 5502: pattern is written with the alternatives in the other order, things are
! 5503: different:
! 5504:
! 5505: ^((.)(?1)\2|.)$
! 5506:
! 5507: This time, the recursing alternative is tried first, and continues to
! 5508: recurse until it runs out of characters, at which point the recursion
! 5509: fails. But this time we do have another alternative to try at the
! 5510: higher level. That is the big difference: in the previous case the
! 5511: remaining alternative is at a deeper recursion level, which PCRE cannot
! 5512: use.
! 5513:
! 5514: To change the pattern so that it matches all palindromic strings, not
! 5515: just those with an odd number of characters, it is tempting to change
! 5516: the pattern to this:
! 5517:
! 5518: ^((.)(?1)\2|.?)$
! 5519:
! 5520: Again, this works in Perl, but not in PCRE, and for the same reason.
! 5521: When a deeper recursion has matched a single character, it cannot be
! 5522: entered again in order to match an empty string. The solution is to
! 5523: separate the two cases, and write out the odd and even cases as alter-
! 5524: natives at the higher level:
! 5525:
! 5526: ^(?:((.)(?1)\2|)|((.)(?3)\4|.))
! 5527:
! 5528: If you want to match typical palindromic phrases, the pattern has to
! 5529: ignore all non-word characters, which can be done like this:
! 5530:
! 5531: ^\W*+(?:((.)\W*+(?1)\W*+\2|)|((.)\W*+(?3)\W*+\4|\W*+.\W*+))\W*+$
! 5532:
! 5533: If run with the PCRE_CASELESS option, this pattern matches phrases such
! 5534: as "A man, a plan, a canal: Panama!" and it works well in both PCRE and
! 5535: Perl. Note the use of the possessive quantifier *+ to avoid backtrack-
! 5536: ing into sequences of non-word characters. Without this, PCRE takes a
! 5537: great deal longer (ten times or more) to match typical phrases, and
! 5538: Perl takes so long that you think it has gone into a loop.
! 5539:
! 5540: WARNING: The palindrome-matching patterns above work only if the sub-
! 5541: ject string does not start with a palindrome that is shorter than the
! 5542: entire string. For example, although "abcba" is correctly matched, if
! 5543: the subject is "ababa", PCRE finds the palindrome "aba" at the start,
! 5544: then fails at top level because the end of the string does not follow.
! 5545: Once again, it cannot jump back into the recursion to try other alter-
! 5546: natives, so the entire match fails.
! 5547:
! 5548: The second way in which PCRE and Perl differ in their recursion pro-
! 5549: cessing is in the handling of captured values. In Perl, when a subpat-
! 5550: tern is called recursively or as a subpattern (see the next section),
! 5551: it has no access to any values that were captured outside the recur-
! 5552: sion, whereas in PCRE these values can be referenced. Consider this
! 5553: pattern:
! 5554:
! 5555: ^(.)(\1|a(?2))
! 5556:
! 5557: In PCRE, this pattern matches "bab". The first capturing parentheses
! 5558: match "b", then in the second group, when the back reference \1 fails
! 5559: to match "b", the second alternative matches "a" and then recurses. In
! 5560: the recursion, \1 does now match "b" and so the whole match succeeds.
! 5561: In Perl, the pattern fails to match because inside the recursive call
! 5562: \1 cannot access the externally set value.
! 5563:
! 5564:
! 5565: SUBPATTERNS AS SUBROUTINES
! 5566:
! 5567: If the syntax for a recursive subpattern call (either by number or by
! 5568: name) is used outside the parentheses to which it refers, it operates
! 5569: like a subroutine in a programming language. The called subpattern may
! 5570: be defined before or after the reference. A numbered reference can be
! 5571: absolute or relative, as in these examples:
! 5572:
! 5573: (...(absolute)...)...(?2)...
! 5574: (...(relative)...)...(?-1)...
! 5575: (...(?+1)...(relative)...
! 5576:
! 5577: An earlier example pointed out that the pattern
! 5578:
! 5579: (sens|respons)e and \1ibility
! 5580:
! 5581: matches "sense and sensibility" and "response and responsibility", but
! 5582: not "sense and responsibility". If instead the pattern
! 5583:
! 5584: (sens|respons)e and (?1)ibility
! 5585:
! 5586: is used, it does match "sense and responsibility" as well as the other
! 5587: two strings. Another example is given in the discussion of DEFINE
! 5588: above.
! 5589:
! 5590: All subroutine calls, whether recursive or not, are always treated as
! 5591: atomic groups. That is, once a subroutine has matched some of the sub-
! 5592: ject string, it is never re-entered, even if it contains untried alter-
! 5593: natives and there is a subsequent matching failure. Any capturing
! 5594: parentheses that are set during the subroutine call revert to their
! 5595: previous values afterwards.
! 5596:
! 5597: Processing options such as case-independence are fixed when a subpat-
! 5598: tern is defined, so if it is used as a subroutine, such options cannot
! 5599: be changed for different calls. For example, consider this pattern:
! 5600:
! 5601: (abc)(?i:(?-1))
! 5602:
! 5603: It matches "abcabc". It does not match "abcABC" because the change of
! 5604: processing option does not affect the called subpattern.
! 5605:
! 5606:
! 5607: ONIGURUMA SUBROUTINE SYNTAX
! 5608:
! 5609: For compatibility with Oniguruma, the non-Perl syntax \g followed by a
! 5610: name or a number enclosed either in angle brackets or single quotes, is
! 5611: an alternative syntax for referencing a subpattern as a subroutine,
! 5612: possibly recursively. Here are two of the examples used above, rewrit-
! 5613: ten using this syntax:
! 5614:
! 5615: (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
! 5616: (sens|respons)e and \g'1'ibility
! 5617:
! 5618: PCRE supports an extension to Oniguruma: if a number is preceded by a
! 5619: plus or a minus sign it is taken as a relative reference. For example:
! 5620:
! 5621: (abc)(?i:\g<-1>)
! 5622:
! 5623: Note that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
! 5624: synonymous. The former is a back reference; the latter is a subroutine
! 5625: call.
! 5626:
! 5627:
! 5628: CALLOUTS
! 5629:
! 5630: Perl has a feature whereby using the sequence (?{...}) causes arbitrary
! 5631: Perl code to be obeyed in the middle of matching a regular expression.
! 5632: This makes it possible, amongst other things, to extract different sub-
! 5633: strings that match the same pair of parentheses when there is a repeti-
! 5634: tion.
! 5635:
! 5636: PCRE provides a similar feature, but of course it cannot obey arbitrary
! 5637: Perl code. The feature is called "callout". The caller of PCRE provides
! 5638: an external function by putting its entry point in the global variable
! 5639: pcre_callout. By default, this variable contains NULL, which disables
! 5640: all calling out.
! 5641:
! 5642: Within a regular expression, (?C) indicates the points at which the
! 5643: external function is to be called. If you want to identify different
! 5644: callout points, you can put a number less than 256 after the letter C.
! 5645: The default value is zero. For example, this pattern has two callout
! 5646: points:
! 5647:
! 5648: (?C1)abc(?C2)def
! 5649:
! 5650: If the PCRE_AUTO_CALLOUT flag is passed to pcre_compile(), callouts are
! 5651: automatically installed before each item in the pattern. They are all
! 5652: numbered 255.
! 5653:
! 5654: During matching, when PCRE reaches a callout point (and pcre_callout is
! 5655: set), the external function is called. It is provided with the number
! 5656: of the callout, the position in the pattern, and, optionally, one item
! 5657: of data originally supplied by the caller of pcre_exec(). The callout
! 5658: function may cause matching to proceed, to backtrack, or to fail alto-
! 5659: gether. A complete description of the interface to the callout function
! 5660: is given in the pcrecallout documentation.
! 5661:
! 5662:
! 5663: BACKTRACKING CONTROL
! 5664:
! 5665: Perl 5.10 introduced a number of "Special Backtracking Control Verbs",
! 5666: which are described in the Perl documentation as "experimental and sub-
! 5667: ject to change or removal in a future version of Perl". It goes on to
! 5668: say: "Their usage in production code should be noted to avoid problems
! 5669: during upgrades." The same remarks apply to the PCRE features described
! 5670: in this section.
! 5671:
! 5672: Since these verbs are specifically related to backtracking, most of
! 5673: them can be used only when the pattern is to be matched using
! 5674: pcre_exec(), which uses a backtracking algorithm. With the exception of
! 5675: (*FAIL), which behaves like a failing negative assertion, they cause an
! 5676: error if encountered by pcre_dfa_exec().
! 5677:
! 5678: If any of these verbs are used in an assertion or in a subpattern that
! 5679: is called as a subroutine (whether or not recursively), their effect is
! 5680: confined to that subpattern; it does not extend to the surrounding pat-
! 5681: tern, with one exception: the name from a *(MARK), (*PRUNE), or (*THEN)
! 5682: that is encountered in a successful positive assertion is passed back
! 5683: when a match succeeds (compare capturing parentheses in assertions).
! 5684: Note that such subpatterns are processed as anchored at the point where
! 5685: they are tested. Note also that Perl's treatment of subroutines is dif-
! 5686: ferent in some cases.
! 5687:
! 5688: The new verbs make use of what was previously invalid syntax: an open-
! 5689: ing parenthesis followed by an asterisk. They are generally of the form
! 5690: (*VERB) or (*VERB:NAME). Some may take either form, with differing be-
! 5691: haviour, depending on whether or not an argument is present. A name is
! 5692: any sequence of characters that does not include a closing parenthesis.
! 5693: If the name is empty, that is, if the closing parenthesis immediately
! 5694: follows the colon, the effect is as if the colon were not there. Any
! 5695: number of these verbs may occur in a pattern.
! 5696:
! 5697: PCRE contains some optimizations that are used to speed up matching by
! 5698: running some checks at the start of each match attempt. For example, it
! 5699: may know the minimum length of matching subject, or that a particular
! 5700: character must be present. When one of these optimizations suppresses
! 5701: the running of a match, any included backtracking verbs will not, of
! 5702: course, be processed. You can suppress the start-of-match optimizations
! 5703: by setting the PCRE_NO_START_OPTIMIZE option when calling pcre_com-
! 5704: pile() or pcre_exec(), or by starting the pattern with (*NO_START_OPT).
! 5705:
! 5706: Experiments with Perl suggest that it too has similar optimizations,
! 5707: sometimes leading to anomalous results.
! 5708:
! 5709: Verbs that act immediately
! 5710:
! 5711: The following verbs act as soon as they are encountered. They may not
! 5712: be followed by a name.
! 5713:
! 5714: (*ACCEPT)
! 5715:
! 5716: This verb causes the match to end successfully, skipping the remainder
! 5717: of the pattern. However, when it is inside a subpattern that is called
! 5718: as a subroutine, only that subpattern is ended successfully. Matching
! 5719: then continues at the outer level. If (*ACCEPT) is inside capturing
! 5720: parentheses, the data so far is captured. For example:
! 5721:
! 5722: A((?:A|B(*ACCEPT)|C)D)
! 5723:
! 5724: This matches "AB", "AAD", or "ACD"; when it matches "AB", "B" is cap-
! 5725: tured by the outer parentheses.
! 5726:
! 5727: (*FAIL) or (*F)
! 5728:
! 5729: This verb causes a matching failure, forcing backtracking to occur. It
! 5730: is equivalent to (?!) but easier to read. The Perl documentation notes
! 5731: that it is probably useful only when combined with (?{}) or (??{}).
! 5732: Those are, of course, Perl features that are not present in PCRE. The
! 5733: nearest equivalent is the callout feature, as for example in this pat-
! 5734: tern:
! 5735:
! 5736: a+(?C)(*FAIL)
! 5737:
! 5738: A match with the string "aaaa" always fails, but the callout is taken
! 5739: before each backtrack happens (in this example, 10 times).
! 5740:
! 5741: Recording which path was taken
! 5742:
! 5743: There is one verb whose main purpose is to track how a match was
! 5744: arrived at, though it also has a secondary use in conjunction with
! 5745: advancing the match starting point (see (*SKIP) below).
! 5746:
! 5747: (*MARK:NAME) or (*:NAME)
! 5748:
! 5749: A name is always required with this verb. There may be as many
! 5750: instances of (*MARK) as you like in a pattern, and their names do not
! 5751: have to be unique.
! 5752:
! 5753: When a match succeeds, the name of the last-encountered (*MARK) on the
! 5754: matching path is passed back to the caller via the pcre_extra data
! 5755: structure, as described in the section on pcre_extra in the pcreapi
! 5756: documentation. Here is an example of pcretest output, where the /K mod-
! 5757: ifier requests the retrieval and outputting of (*MARK) data:
! 5758:
! 5759: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
! 5760: data> XY
! 5761: 0: XY
! 5762: MK: A
! 5763: XZ
! 5764: 0: XZ
! 5765: MK: B
! 5766:
! 5767: The (*MARK) name is tagged with "MK:" in this output, and in this exam-
! 5768: ple it indicates which of the two alternatives matched. This is a more
! 5769: efficient way of obtaining this information than putting each alterna-
! 5770: tive in its own capturing parentheses.
! 5771:
! 5772: If (*MARK) is encountered in a positive assertion, its name is recorded
! 5773: and passed back if it is the last-encountered. This does not happen for
! 5774: negative assertions.
! 5775:
! 5776: After a partial match or a failed match, the name of the last encoun-
! 5777: tered (*MARK) in the entire match process is returned. For example:
! 5778:
! 5779: re> /X(*MARK:A)Y|X(*MARK:B)Z/K
! 5780: data> XP
! 5781: No match, mark = B
! 5782:
! 5783: Note that in this unanchored example the mark is retained from the
! 5784: match attempt that started at the letter "X". Subsequent match attempts
! 5785: starting at "P" and then with an empty string do not get as far as the
! 5786: (*MARK) item, but nevertheless do not reset it.
! 5787:
! 5788: Verbs that act after backtracking
! 5789:
! 5790: The following verbs do nothing when they are encountered. Matching con-
! 5791: tinues with what follows, but if there is no subsequent match, causing
! 5792: a backtrack to the verb, a failure is forced. That is, backtracking
! 5793: cannot pass to the left of the verb. However, when one of these verbs
! 5794: appears inside an atomic group, its effect is confined to that group,
! 5795: because once the group has been matched, there is never any backtrack-
! 5796: ing into it. In this situation, backtracking can "jump back" to the
! 5797: left of the entire atomic group. (Remember also, as stated above, that
! 5798: this localization also applies in subroutine calls and assertions.)
! 5799:
! 5800: These verbs differ in exactly what kind of failure occurs when back-
! 5801: tracking reaches them.
! 5802:
! 5803: (*COMMIT)
! 5804:
! 5805: This verb, which may not be followed by a name, causes the whole match
! 5806: to fail outright if the rest of the pattern does not match. Even if the
! 5807: pattern is unanchored, no further attempts to find a match by advancing
! 5808: the starting point take place. Once (*COMMIT) has been passed,
! 5809: pcre_exec() is committed to finding a match at the current starting
! 5810: point, or not at all. For example:
! 5811:
! 5812: a+(*COMMIT)b
! 5813:
! 5814: This matches "xxaab" but not "aacaab". It can be thought of as a kind
! 5815: of dynamic anchor, or "I've started, so I must finish." The name of the
! 5816: most recently passed (*MARK) in the path is passed back when (*COMMIT)
! 5817: forces a match failure.
! 5818:
! 5819: Note that (*COMMIT) at the start of a pattern is not the same as an
! 5820: anchor, unless PCRE's start-of-match optimizations are turned off, as
! 5821: shown in this pcretest example:
! 5822:
! 5823: re> /(*COMMIT)abc/
! 5824: data> xyzabc
! 5825: 0: abc
! 5826: xyzabc\Y
! 5827: No match
! 5828:
! 5829: PCRE knows that any match must start with "a", so the optimization
! 5830: skips along the subject to "a" before running the first match attempt,
! 5831: which succeeds. When the optimization is disabled by the \Y escape in
! 5832: the second subject, the match starts at "x" and so the (*COMMIT) causes
! 5833: it to fail without trying any other starting points.
! 5834:
! 5835: (*PRUNE) or (*PRUNE:NAME)
! 5836:
! 5837: This verb causes the match to fail at the current starting position in
! 5838: the subject if the rest of the pattern does not match. If the pattern
! 5839: is unanchored, the normal "bumpalong" advance to the next starting
! 5840: character then happens. Backtracking can occur as usual to the left of
! 5841: (*PRUNE), before it is reached, or when matching to the right of
! 5842: (*PRUNE), but if there is no match to the right, backtracking cannot
! 5843: cross (*PRUNE). In simple cases, the use of (*PRUNE) is just an alter-
! 5844: native to an atomic group or possessive quantifier, but there are some
! 5845: uses of (*PRUNE) that cannot be expressed in any other way. The behav-
! 5846: iour of (*PRUNE:NAME) is the same as (*MARK:NAME)(*PRUNE). In an
! 5847: anchored pattern (*PRUNE) has the same effect as (*COMMIT).
! 5848:
! 5849: (*SKIP)
! 5850:
! 5851: This verb, when given without a name, is like (*PRUNE), except that if
! 5852: the pattern is unanchored, the "bumpalong" advance is not to the next
! 5853: character, but to the position in the subject where (*SKIP) was encoun-
! 5854: tered. (*SKIP) signifies that whatever text was matched leading up to
! 5855: it cannot be part of a successful match. Consider:
! 5856:
! 5857: a+(*SKIP)b
! 5858:
! 5859: If the subject is "aaaac...", after the first match attempt fails
! 5860: (starting at the first character in the string), the starting point
! 5861: skips on to start the next attempt at "c". Note that a possessive quan-
! 5862: tifer does not have the same effect as this example; although it would
! 5863: suppress backtracking during the first match attempt, the second
! 5864: attempt would start at the second character instead of skipping on to
! 5865: "c".
! 5866:
! 5867: (*SKIP:NAME)
! 5868:
! 5869: When (*SKIP) has an associated name, its behaviour is modified. If the
! 5870: following pattern fails to match, the previous path through the pattern
! 5871: is searched for the most recent (*MARK) that has the same name. If one
! 5872: is found, the "bumpalong" advance is to the subject position that cor-
! 5873: responds to that (*MARK) instead of to where (*SKIP) was encountered.
! 5874: If no (*MARK) with a matching name is found, the (*SKIP) is ignored.
! 5875:
! 5876: (*THEN) or (*THEN:NAME)
! 5877:
! 5878: This verb causes a skip to the next innermost alternative if the rest
! 5879: of the pattern does not match. That is, it cancels pending backtrack-
! 5880: ing, but only within the current alternative. Its name comes from the
! 5881: observation that it can be used for a pattern-based if-then-else block:
! 5882:
! 5883: ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...
! 5884:
! 5885: If the COND1 pattern matches, FOO is tried (and possibly further items
! 5886: after the end of the group if FOO succeeds); on failure, the matcher
! 5887: skips to the second alternative and tries COND2, without backtracking
! 5888: into COND1. The behaviour of (*THEN:NAME) is exactly the same as
! 5889: (*MARK:NAME)(*THEN). If (*THEN) is not inside an alternation, it acts
! 5890: like (*PRUNE).
! 5891:
! 5892: Note that a subpattern that does not contain a | character is just a
! 5893: part of the enclosing alternative; it is not a nested alternation with
! 5894: only one alternative. The effect of (*THEN) extends beyond such a sub-
! 5895: pattern to the enclosing alternative. Consider this pattern, where A,
! 5896: B, etc. are complex pattern fragments that do not contain any | charac-
! 5897: ters at this level:
! 5898:
! 5899: A (B(*THEN)C) | D
! 5900:
! 5901: If A and B are matched, but there is a failure in C, matching does not
! 5902: backtrack into A; instead it moves to the next alternative, that is, D.
! 5903: However, if the subpattern containing (*THEN) is given an alternative,
! 5904: it behaves differently:
! 5905:
! 5906: A (B(*THEN)C | (*FAIL)) | D
! 5907:
! 5908: The effect of (*THEN) is now confined to the inner subpattern. After a
! 5909: failure in C, matching moves to (*FAIL), which causes the whole subpat-
! 5910: tern to fail because there are no more alternatives to try. In this
! 5911: case, matching does now backtrack into A.
! 5912:
! 5913: Note also that a conditional subpattern is not considered as having two
! 5914: alternatives, because only one is ever used. In other words, the |
! 5915: character in a conditional subpattern has a different meaning. Ignoring
! 5916: white space, consider:
! 5917:
! 5918: ^.*? (?(?=a) a | b(*THEN)c )
! 5919:
! 5920: If the subject is "ba", this pattern does not match. Because .*? is
! 5921: ungreedy, it initially matches zero characters. The condition (?=a)
! 5922: then fails, the character "b" is matched, but "c" is not. At this
! 5923: point, matching does not backtrack to .*? as might perhaps be expected
! 5924: from the presence of the | character. The conditional subpattern is
! 5925: part of the single alternative that comprises the whole pattern, and so
! 5926: the match fails. (If there was a backtrack into .*?, allowing it to
! 5927: match "b", the match would succeed.)
! 5928:
! 5929: The verbs just described provide four different "strengths" of control
! 5930: when subsequent matching fails. (*THEN) is the weakest, carrying on the
! 5931: match at the next alternative. (*PRUNE) comes next, failing the match
! 5932: at the current starting position, but allowing an advance to the next
! 5933: character (for an unanchored pattern). (*SKIP) is similar, except that
! 5934: the advance may be more than one character. (*COMMIT) is the strongest,
! 5935: causing the entire match to fail.
! 5936:
! 5937: If more than one such verb is present in a pattern, the "strongest" one
! 5938: wins. For example, consider this pattern, where A, B, etc. are complex
! 5939: pattern fragments:
! 5940:
! 5941: (A(*COMMIT)B(*THEN)C|D)
! 5942:
! 5943: Once A has matched, PCRE is committed to this match, at the current
! 5944: starting position. If subsequently B matches, but C does not, the nor-
! 5945: mal (*THEN) action of trying the next alternative (that is, D) does not
! 5946: happen because (*COMMIT) overrides.
! 5947:
! 5948:
! 5949: SEE ALSO
! 5950:
! 5951: pcreapi(3), pcrecallout(3), pcrematching(3), pcresyntax(3), pcre(3).
! 5952:
! 5953:
! 5954: AUTHOR
! 5955:
! 5956: Philip Hazel
! 5957: University Computing Service
! 5958: Cambridge CB2 3QH, England.
! 5959:
! 5960:
! 5961: REVISION
! 5962:
! 5963: Last updated: 29 November 2011
! 5964: Copyright (c) 1997-2011 University of Cambridge.
! 5965: ------------------------------------------------------------------------------
! 5966:
! 5967:
! 5968: PCRESYNTAX(3) PCRESYNTAX(3)
! 5969:
! 5970:
! 5971: NAME
! 5972: PCRE - Perl-compatible regular expressions
! 5973:
! 5974:
! 5975: PCRE REGULAR EXPRESSION SYNTAX SUMMARY
! 5976:
! 5977: The full syntax and semantics of the regular expressions that are sup-
! 5978: ported by PCRE are described in the pcrepattern documentation. This
! 5979: document contains just a quick-reference summary of the syntax.
! 5980:
! 5981:
! 5982: QUOTING
! 5983:
! 5984: \x where x is non-alphanumeric is a literal x
! 5985: \Q...\E treat enclosed characters as literal
! 5986:
! 5987:
! 5988: CHARACTERS
! 5989:
! 5990: \a alarm, that is, the BEL character (hex 07)
! 5991: \cx "control-x", where x is any ASCII character
! 5992: \e escape (hex 1B)
! 5993: \f formfeed (hex 0C)
! 5994: \n newline (hex 0A)
! 5995: \r carriage return (hex 0D)
! 5996: \t tab (hex 09)
! 5997: \ddd character with octal code ddd, or backreference
! 5998: \xhh character with hex code hh
! 5999: \x{hhh..} character with hex code hhh..
! 6000:
! 6001:
! 6002: CHARACTER TYPES
! 6003:
! 6004: . any character except newline;
! 6005: in dotall mode, any character whatsoever
! 6006: \C one byte, even in UTF-8 mode (best avoided)
! 6007: \d a decimal digit
! 6008: \D a character that is not a decimal digit
! 6009: \h a horizontal whitespace character
! 6010: \H a character that is not a horizontal whitespace character
! 6011: \N a character that is not a newline
! 6012: \p{xx} a character with the xx property
! 6013: \P{xx} a character without the xx property
! 6014: \R a newline sequence
! 6015: \s a whitespace character
! 6016: \S a character that is not a whitespace character
! 6017: \v a vertical whitespace character
! 6018: \V a character that is not a vertical whitespace character
! 6019: \w a "word" character
! 6020: \W a "non-word" character
! 6021: \X an extended Unicode sequence
! 6022:
! 6023: In PCRE, by default, \d, \D, \s, \S, \w, and \W recognize only ASCII
! 6024: characters, even in UTF-8 mode. However, this can be changed by setting
! 6025: the PCRE_UCP option.
! 6026:
! 6027:
! 6028: GENERAL CATEGORY PROPERTIES FOR \p and \P
! 6029:
! 6030: C Other
! 6031: Cc Control
! 6032: Cf Format
! 6033: Cn Unassigned
! 6034: Co Private use
! 6035: Cs Surrogate
! 6036:
! 6037: L Letter
! 6038: Ll Lower case letter
! 6039: Lm Modifier letter
! 6040: Lo Other letter
! 6041: Lt Title case letter
! 6042: Lu Upper case letter
! 6043: L& Ll, Lu, or Lt
! 6044:
! 6045: M Mark
! 6046: Mc Spacing mark
! 6047: Me Enclosing mark
! 6048: Mn Non-spacing mark
! 6049:
! 6050: N Number
! 6051: Nd Decimal number
! 6052: Nl Letter number
! 6053: No Other number
! 6054:
! 6055: P Punctuation
! 6056: Pc Connector punctuation
! 6057: Pd Dash punctuation
! 6058: Pe Close punctuation
! 6059: Pf Final punctuation
! 6060: Pi Initial punctuation
! 6061: Po Other punctuation
! 6062: Ps Open punctuation
! 6063:
! 6064: S Symbol
! 6065: Sc Currency symbol
! 6066: Sk Modifier symbol
! 6067: Sm Mathematical symbol
! 6068: So Other symbol
! 6069:
! 6070: Z Separator
! 6071: Zl Line separator
! 6072: Zp Paragraph separator
! 6073: Zs Space separator
! 6074:
! 6075:
! 6076: PCRE SPECIAL CATEGORY PROPERTIES FOR \p and \P
! 6077:
! 6078: Xan Alphanumeric: union of properties L and N
! 6079: Xps POSIX space: property Z or tab, NL, VT, FF, CR
! 6080: Xsp Perl space: property Z or tab, NL, FF, CR
! 6081: Xwd Perl word: property Xan or underscore
! 6082:
! 6083:
! 6084: SCRIPT NAMES FOR \p AND \P
! 6085:
! 6086: Arabic, Armenian, Avestan, Balinese, Bamum, Bengali, Bopomofo, Braille,
! 6087: Buginese, Buhid, Canadian_Aboriginal, Carian, Cham, Cherokee, Common,
! 6088: Coptic, Cuneiform, Cypriot, Cyrillic, Deseret, Devanagari, Egyp-
! 6089: tian_Hieroglyphs, Ethiopic, Georgian, Glagolitic, Gothic, Greek,
! 6090: Gujarati, Gurmukhi, Han, Hangul, Hanunoo, Hebrew, Hiragana, Impe-
! 6091: rial_Aramaic, Inherited, Inscriptional_Pahlavi, Inscriptional_Parthian,
! 6092: Javanese, Kaithi, Kannada, Katakana, Kayah_Li, Kharoshthi, Khmer, Lao,
! 6093: Latin, Lepcha, Limbu, Linear_B, Lisu, Lycian, Lydian, Malayalam,
! 6094: Meetei_Mayek, Mongolian, Myanmar, New_Tai_Lue, Nko, Ogham, Old_Italic,
! 6095: Old_Persian, Old_South_Arabian, Old_Turkic, Ol_Chiki, Oriya, Osmanya,
! 6096: Phags_Pa, Phoenician, Rejang, Runic, Samaritan, Saurashtra, Shavian,
! 6097: Sinhala, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa, Tai_Le,
! 6098: Tai_Tham, Tai_Viet, Tamil, Telugu, Thaana, Thai, Tibetan, Tifinagh,
! 6099: Ugaritic, Vai, Yi.
! 6100:
! 6101:
! 6102: CHARACTER CLASSES
! 6103:
! 6104: [...] positive character class
! 6105: [^...] negative character class
! 6106: [x-y] range (can be used for hex characters)
! 6107: [[:xxx:]] positive POSIX named set
! 6108: [[:^xxx:]] negative POSIX named set
! 6109:
! 6110: alnum alphanumeric
! 6111: alpha alphabetic
! 6112: ascii 0-127
! 6113: blank space or tab
! 6114: cntrl control character
! 6115: digit decimal digit
! 6116: graph printing, excluding space
! 6117: lower lower case letter
! 6118: print printing, including space
! 6119: punct printing, excluding alphanumeric
! 6120: space whitespace
! 6121: upper upper case letter
! 6122: word same as \w
! 6123: xdigit hexadecimal digit
! 6124:
! 6125: In PCRE, POSIX character set names recognize only ASCII characters by
! 6126: default, but some of them use Unicode properties if PCRE_UCP is set.
! 6127: You can use \Q...\E inside a character class.
! 6128:
! 6129:
! 6130: QUANTIFIERS
! 6131:
! 6132: ? 0 or 1, greedy
! 6133: ?+ 0 or 1, possessive
! 6134: ?? 0 or 1, lazy
! 6135: * 0 or more, greedy
! 6136: *+ 0 or more, possessive
! 6137: *? 0 or more, lazy
! 6138: + 1 or more, greedy
! 6139: ++ 1 or more, possessive
! 6140: +? 1 or more, lazy
! 6141: {n} exactly n
! 6142: {n,m} at least n, no more than m, greedy
! 6143: {n,m}+ at least n, no more than m, possessive
! 6144: {n,m}? at least n, no more than m, lazy
! 6145: {n,} n or more, greedy
! 6146: {n,}+ n or more, possessive
! 6147: {n,}? n or more, lazy
! 6148:
! 6149:
! 6150: ANCHORS AND SIMPLE ASSERTIONS
! 6151:
! 6152: \b word boundary
! 6153: \B not a word boundary
! 6154: ^ start of subject
! 6155: also after internal newline in multiline mode
! 6156: \A start of subject
! 6157: $ end of subject
! 6158: also before newline at end of subject
! 6159: also before internal newline in multiline mode
! 6160: \Z end of subject
! 6161: also before newline at end of subject
! 6162: \z end of subject
! 6163: \G first matching position in subject
! 6164:
! 6165:
! 6166: MATCH POINT RESET
! 6167:
! 6168: \K reset start of match
! 6169:
! 6170:
! 6171: ALTERNATION
! 6172:
! 6173: expr|expr|expr...
! 6174:
! 6175:
! 6176: CAPTURING
! 6177:
! 6178: (...) capturing group
! 6179: (?<name>...) named capturing group (Perl)
! 6180: (?'name'...) named capturing group (Perl)
! 6181: (?P<name>...) named capturing group (Python)
! 6182: (?:...) non-capturing group
! 6183: (?|...) non-capturing group; reset group numbers for
! 6184: capturing groups in each alternative
! 6185:
! 6186:
! 6187: ATOMIC GROUPS
! 6188:
! 6189: (?>...) atomic, non-capturing group
! 6190:
! 6191:
! 6192: COMMENT
! 6193:
! 6194: (?#....) comment (not nestable)
! 6195:
! 6196:
! 6197: OPTION SETTING
! 6198:
! 6199: (?i) caseless
! 6200: (?J) allow duplicate names
! 6201: (?m) multiline
! 6202: (?s) single line (dotall)
! 6203: (?U) default ungreedy (lazy)
! 6204: (?x) extended (ignore white space)
! 6205: (?-...) unset option(s)
! 6206:
! 6207: The following are recognized only at the start of a pattern or after
! 6208: one of the newline-setting options with similar syntax:
! 6209:
! 6210: (*NO_START_OPT) no start-match optimization (PCRE_NO_START_OPTIMIZE)
! 6211: (*UTF8) set UTF-8 mode (PCRE_UTF8)
! 6212: (*UCP) set PCRE_UCP (use Unicode properties for \d etc)
! 6213:
! 6214:
! 6215: LOOKAHEAD AND LOOKBEHIND ASSERTIONS
! 6216:
! 6217: (?=...) positive look ahead
! 6218: (?!...) negative look ahead
! 6219: (?<=...) positive look behind
! 6220: (?<!...) negative look behind
! 6221:
! 6222: Each top-level branch of a look behind must be of a fixed length.
! 6223:
! 6224:
! 6225: BACKREFERENCES
! 6226:
! 6227: \n reference by number (can be ambiguous)
! 6228: \gn reference by number
! 6229: \g{n} reference by number
! 6230: \g{-n} relative reference by number
! 6231: \k<name> reference by name (Perl)
! 6232: \k'name' reference by name (Perl)
! 6233: \g{name} reference by name (Perl)
! 6234: \k{name} reference by name (.NET)
! 6235: (?P=name) reference by name (Python)
! 6236:
! 6237:
! 6238: SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)
! 6239:
! 6240: (?R) recurse whole pattern
! 6241: (?n) call subpattern by absolute number
! 6242: (?+n) call subpattern by relative number
! 6243: (?-n) call subpattern by relative number
! 6244: (?&name) call subpattern by name (Perl)
! 6245: (?P>name) call subpattern by name (Python)
! 6246: \g<name> call subpattern by name (Oniguruma)
! 6247: \g'name' call subpattern by name (Oniguruma)
! 6248: \g<n> call subpattern by absolute number (Oniguruma)
! 6249: \g'n' call subpattern by absolute number (Oniguruma)
! 6250: \g<+n> call subpattern by relative number (PCRE extension)
! 6251: \g'+n' call subpattern by relative number (PCRE extension)
! 6252: \g<-n> call subpattern by relative number (PCRE extension)
! 6253: \g'-n' call subpattern by relative number (PCRE extension)
! 6254:
! 6255:
! 6256: CONDITIONAL PATTERNS
! 6257:
! 6258: (?(condition)yes-pattern)
! 6259: (?(condition)yes-pattern|no-pattern)
! 6260:
! 6261: (?(n)... absolute reference condition
! 6262: (?(+n)... relative reference condition
! 6263: (?(-n)... relative reference condition
! 6264: (?(<name>)... named reference condition (Perl)
! 6265: (?('name')... named reference condition (Perl)
! 6266: (?(name)... named reference condition (PCRE)
! 6267: (?(R)... overall recursion condition
! 6268: (?(Rn)... specific group recursion condition
! 6269: (?(R&name)... specific recursion condition
! 6270: (?(DEFINE)... define subpattern for reference
! 6271: (?(assert)... assertion condition
! 6272:
! 6273:
! 6274: BACKTRACKING CONTROL
! 6275:
! 6276: The following act immediately they are reached:
! 6277:
! 6278: (*ACCEPT) force successful match
! 6279: (*FAIL) force backtrack; synonym (*F)
! 6280:
! 6281: The following act only when a subsequent match failure causes a back-
! 6282: track to reach them. They all force a match failure, but they differ in
! 6283: what happens afterwards. Those that advance the start-of-match point do
! 6284: so only if the pattern is not anchored.
! 6285:
! 6286: (*COMMIT) overall failure, no advance of starting point
! 6287: (*PRUNE) advance to next starting character
! 6288: (*SKIP) advance start to current matching position
! 6289: (*THEN) local failure, backtrack to next alternation
! 6290:
! 6291:
! 6292: NEWLINE CONVENTIONS
! 6293:
! 6294: These are recognized only at the very start of the pattern or after a
! 6295: (*BSR_...) or (*UTF8) or (*UCP) option.
! 6296:
! 6297: (*CR) carriage return only
! 6298: (*LF) linefeed only
! 6299: (*CRLF) carriage return followed by linefeed
! 6300: (*ANYCRLF) all three of the above
! 6301: (*ANY) any Unicode newline sequence
! 6302:
! 6303:
! 6304: WHAT \R MATCHES
! 6305:
! 6306: These are recognized only at the very start of the pattern or after a
! 6307: (*...) option that sets the newline convention or UTF-8 or UCP mode.
! 6308:
! 6309: (*BSR_ANYCRLF) CR, LF, or CRLF
! 6310: (*BSR_UNICODE) any Unicode newline sequence
! 6311:
! 6312:
! 6313: CALLOUTS
! 6314:
! 6315: (?C) callout
! 6316: (?Cn) callout with data n
! 6317:
! 6318:
! 6319: SEE ALSO
! 6320:
! 6321: pcrepattern(3), pcreapi(3), pcrecallout(3), pcrematching(3), pcre(3).
! 6322:
! 6323:
! 6324: AUTHOR
! 6325:
! 6326: Philip Hazel
! 6327: University Computing Service
! 6328: Cambridge CB2 3QH, England.
! 6329:
! 6330:
! 6331: REVISION
! 6332:
! 6333: Last updated: 21 November 2010
! 6334: Copyright (c) 1997-2010 University of Cambridge.
! 6335: ------------------------------------------------------------------------------
! 6336:
! 6337:
! 6338: PCREUNICODE(3) PCREUNICODE(3)
! 6339:
! 6340:
! 6341: NAME
! 6342: PCRE - Perl-compatible regular expressions
! 6343:
! 6344:
! 6345: UTF-8 AND UNICODE PROPERTY SUPPORT
! 6346:
! 6347: In order process UTF-8 strings, you must build PCRE to include UTF-8
! 6348: support in the code, and, in addition, you must call pcre_compile()
! 6349: with the PCRE_UTF8 option flag, or the pattern must start with the
! 6350: sequence (*UTF8). When either of these is the case, both the pattern
! 6351: and any subject strings that are matched against it are treated as
! 6352: UTF-8 strings instead of strings of 1-byte characters. PCRE does not
! 6353: support any other formats (in particular, it does not support UTF-16).
! 6354:
! 6355: If you compile PCRE with UTF-8 support, but do not use it at run time,
! 6356: the library will be a bit bigger, but the additional run time overhead
! 6357: is limited to testing the PCRE_UTF8 flag occasionally, so should not be
! 6358: very big.
! 6359:
! 6360: If PCRE is built with Unicode character property support (which implies
! 6361: UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup-
! 6362: ported. The available properties that can be tested are limited to the
! 6363: general category properties such as Lu for an upper case letter or Nd
! 6364: for a decimal number, the Unicode script names such as Arabic or Han,
! 6365: and the derived properties Any and L&. A full list is given in the
! 6366: pcrepattern documentation. Only the short names for properties are sup-
! 6367: ported. For example, \p{L} matches a letter. Its Perl synonym, \p{Let-
! 6368: ter}, is not supported. Furthermore, in Perl, many properties may
! 6369: optionally be prefixed by "Is", for compatibility with Perl 5.6. PCRE
! 6370: does not support this.
! 6371:
! 6372: Validity of UTF-8 strings
! 6373:
! 6374: When you set the PCRE_UTF8 flag, the strings passed as patterns and
! 6375: subjects are (by default) checked for validity on entry to the relevant
! 6376: functions. From release 7.3 of PCRE, the check is according the rules
! 6377: of RFC 3629, which are themselves derived from the Unicode specifica-
! 6378: tion. Earlier releases of PCRE followed the rules of RFC 2279, which
! 6379: allows the full range of 31-bit values (0 to 0x7FFFFFFF). The current
! 6380: check allows only values in the range U+0 to U+10FFFF, excluding U+D800
! 6381: to U+DFFF.
! 6382:
! 6383: The excluded code points are the "Low Surrogate Area" of Unicode, of
! 6384: which the Unicode Standard says this: "The Low Surrogate Area does not
! 6385: contain any character assignments, consequently no character code
! 6386: charts or namelists are provided for this area. Surrogates are reserved
! 6387: for use with UTF-16 and then must be used in pairs." The code points
! 6388: that are encoded by UTF-16 pairs are available as independent code
! 6389: points in the UTF-8 encoding. (In other words, the whole surrogate
! 6390: thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
! 6391:
! 6392: If an invalid UTF-8 string is passed to PCRE, an error return is given.
! 6393: At compile time, the only additional information is the offset to the
! 6394: first byte of the failing character. The runtime functions pcre_exec()
! 6395: and pcre_dfa_exec() also pass back this information, as well as a more
! 6396: detailed reason code if the caller has provided memory in which to do
! 6397: this.
! 6398:
! 6399: In some situations, you may already know that your strings are valid,
! 6400: and therefore want to skip these checks in order to improve perfor-
! 6401: mance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or at run
! 6402: time, PCRE assumes that the pattern or subject it is given (respec-
! 6403: tively) contains only valid UTF-8 codes. In this case, it does not
! 6404: diagnose an invalid UTF-8 string.
! 6405:
! 6406: If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set,
! 6407: what happens depends on why the string is invalid. If the string con-
! 6408: forms to the "old" definition of UTF-8 (RFC 2279), it is processed as a
! 6409: string of characters in the range 0 to 0x7FFFFFFF by pcre_dfa_exec()
! 6410: and the interpreted version of pcre_exec(). In other words, apart from
! 6411: the initial validity test, these functions (when in UTF-8 mode) handle
! 6412: strings according to the more liberal rules of RFC 2279. However, the
! 6413: just-in-time (JIT) optimization for pcre_exec() supports only RFC 3629.
! 6414: If you are using JIT optimization, or if the string does not even con-
! 6415: form to RFC 2279, the result is undefined. Your program may crash.
! 6416:
! 6417: If you want to process strings of values in the full range 0 to
! 6418: 0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can
! 6419: set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
! 6420: this situation, you will have to apply your own validity check, and
! 6421: avoid the use of JIT optimization.
! 6422:
! 6423: General comments about UTF-8 mode
! 6424:
! 6425: 1. An unbraced hexadecimal escape sequence (such as \xb3) matches a
! 6426: two-byte UTF-8 character if the value is greater than 127.
! 6427:
! 6428: 2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
! 6429: characters for values greater than \177.
! 6430:
! 6431: 3. Repeat quantifiers apply to complete UTF-8 characters, not to indi-
! 6432: vidual bytes, for example: \x{100}{3}.
! 6433:
! 6434: 4. The dot metacharacter matches one UTF-8 character instead of a sin-
! 6435: gle byte.
! 6436:
! 6437: 5. The escape sequence \C can be used to match a single byte in UTF-8
! 6438: mode, but its use can lead to some strange effects because it breaks up
! 6439: multibyte characters (see the description of \C in the pcrepattern doc-
! 6440: umentation). The use of \C is not supported in the alternative matching
! 6441: function pcre_dfa_exec(), nor is it supported in UTF-8 mode by the JIT
! 6442: optimization of pcre_exec(). If JIT optimization is requested for a
! 6443: UTF-8 pattern that contains \C, it will not succeed, and so the match-
! 6444: ing will be carried out by the normal interpretive function.
! 6445:
! 6446: 6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
! 6447: test characters of any code value, but, by default, the characters that
! 6448: PCRE recognizes as digits, spaces, or word characters remain the same
! 6449: set as before, all with values less than 256. This remains true even
! 6450: when PCRE is built to include Unicode property support, because to do
! 6451: otherwise would slow down PCRE in many common cases. Note in particular
! 6452: that this applies to \b and \B, because they are defined in terms of \w
! 6453: and \W. If you really want to test for a wider sense of, say, "digit",
! 6454: you can use explicit Unicode property tests such as \p{Nd}. Alterna-
! 6455: tively, if you set the PCRE_UCP option, the way that the character
! 6456: escapes work is changed so that Unicode properties are used to deter-
! 6457: mine which characters match. There are more details in the section on
! 6458: generic character types in the pcrepattern documentation.
! 6459:
! 6460: 7. Similarly, characters that match the POSIX named character classes
! 6461: are all low-valued characters, unless the PCRE_UCP option is set.
! 6462:
! 6463: 8. However, the horizontal and vertical whitespace matching escapes
! 6464: (\h, \H, \v, and \V) do match all the appropriate Unicode characters,
! 6465: whether or not PCRE_UCP is set.
! 6466:
! 6467: 9. Case-insensitive matching applies only to characters whose values
! 6468: are less than 128, unless PCRE is built with Unicode property support.
! 6469: Even when Unicode property support is available, PCRE still uses its
! 6470: own character tables when checking the case of low-valued characters,
! 6471: so as not to degrade performance. The Unicode property information is
! 6472: used only for characters with higher values. Furthermore, PCRE supports
! 6473: case-insensitive matching only when there is a one-to-one mapping
! 6474: between a letter's cases. There are a small number of many-to-one map-
! 6475: pings in Unicode; these are not supported by PCRE.
! 6476:
! 6477:
! 6478: AUTHOR
! 6479:
! 6480: Philip Hazel
! 6481: University Computing Service
! 6482: Cambridge CB2 3QH, England.
! 6483:
! 6484:
! 6485: REVISION
! 6486:
! 6487: Last updated: 19 October 2011
! 6488: Copyright (c) 1997-2011 University of Cambridge.
! 6489: ------------------------------------------------------------------------------
! 6490:
! 6491:
! 6492: PCREJIT(3) PCREJIT(3)
! 6493:
! 6494:
! 6495: NAME
! 6496: PCRE - Perl-compatible regular expressions
! 6497:
! 6498:
! 6499: PCRE JUST-IN-TIME COMPILER SUPPORT
! 6500:
! 6501: Just-in-time compiling is a heavyweight optimization that can greatly
! 6502: speed up pattern matching. However, it comes at the cost of extra pro-
! 6503: cessing before the match is performed. Therefore, it is of most benefit
! 6504: when the same pattern is going to be matched many times. This does not
! 6505: necessarily mean many calls of pcre_exec(); if the pattern is not
! 6506: anchored, matching attempts may take place many times at various posi-
! 6507: tions in the subject, even for a single call to pcre_exec(). If the
! 6508: subject string is very long, it may still pay to use JIT for one-off
! 6509: matches.
! 6510:
! 6511: JIT support applies only to the traditional matching function,
! 6512: pcre_exec(). It does not apply when pcre_dfa_exec() is being used. The
! 6513: code for this support was written by Zoltan Herczeg.
! 6514:
! 6515:
! 6516: AVAILABILITY OF JIT SUPPORT
! 6517:
! 6518: JIT support is an optional feature of PCRE. The "configure" option
! 6519: --enable-jit (or equivalent CMake option) must be set when PCRE is
! 6520: built if you want to use JIT. The support is limited to the following
! 6521: hardware platforms:
! 6522:
! 6523: ARM v5, v7, and Thumb2
! 6524: Intel x86 32-bit and 64-bit
! 6525: MIPS 32-bit
! 6526: Power PC 32-bit and 64-bit (experimental)
! 6527:
! 6528: The Power PC support is designated as experimental because it has not
! 6529: been fully tested. If --enable-jit is set on an unsupported platform,
! 6530: compilation fails.
! 6531:
! 6532: A program that is linked with PCRE 8.20 or later can tell if JIT sup-
! 6533: port is available by calling pcre_config() with the PCRE_CONFIG_JIT
! 6534: option. The result is 1 when JIT is available, and 0 otherwise. How-
! 6535: ever, a simple program does not need to check this in order to use JIT.
! 6536: The API is implemented in a way that falls back to the ordinary PCRE
! 6537: code if JIT is not available.
! 6538:
! 6539: If your program may sometimes be linked with versions of PCRE that are
! 6540: older than 8.20, but you want to use JIT when it is available, you can
! 6541: test the values of PCRE_MAJOR and PCRE_MINOR, or the existence of a JIT
! 6542: macro such as PCRE_CONFIG_JIT, for compile-time control of your code.
! 6543:
! 6544:
! 6545: SIMPLE USE OF JIT
! 6546:
! 6547: You have to do two things to make use of the JIT support in the sim-
! 6548: plest way:
! 6549:
! 6550: (1) Call pcre_study() with the PCRE_STUDY_JIT_COMPILE option for
! 6551: each compiled pattern, and pass the resulting pcre_extra block to
! 6552: pcre_exec().
! 6553:
! 6554: (2) Use pcre_free_study() to free the pcre_extra block when it is
! 6555: no longer needed instead of just freeing it yourself. This
! 6556: ensures that any JIT data is also freed.
! 6557:
! 6558: For a program that may be linked with pre-8.20 versions of PCRE, you
! 6559: can insert
! 6560:
! 6561: #ifndef PCRE_STUDY_JIT_COMPILE
! 6562: #define PCRE_STUDY_JIT_COMPILE 0
! 6563: #endif
! 6564:
! 6565: so that no option is passed to pcre_study(), and then use something
! 6566: like this to free the study data:
! 6567:
! 6568: #ifdef PCRE_CONFIG_JIT
! 6569: pcre_free_study(study_ptr);
! 6570: #else
! 6571: pcre_free(study_ptr);
! 6572: #endif
! 6573:
! 6574: In some circumstances you may need to call additional functions. These
! 6575: are described in the section entitled "Controlling the JIT stack"
! 6576: below.
! 6577:
! 6578: If JIT support is not available, PCRE_STUDY_JIT_COMPILE is ignored, and
! 6579: no JIT data is set up. Otherwise, the compiled pattern is passed to the
! 6580: JIT compiler, which turns it into machine code that executes much
! 6581: faster than the normal interpretive code. When pcre_exec() is passed a
! 6582: pcre_extra block containing a pointer to JIT code, it obeys that
! 6583: instead of the normal code. The result is identical, but the code runs
! 6584: much faster.
! 6585:
! 6586: There are some pcre_exec() options that are not supported for JIT exe-
! 6587: cution. There are also some pattern items that JIT cannot handle.
! 6588: Details are given below. In both cases, execution automatically falls
! 6589: back to the interpretive code.
! 6590:
! 6591: If the JIT compiler finds an unsupported item, no JIT data is gener-
! 6592: ated. You can find out if JIT execution is available after studying a
! 6593: pattern by calling pcre_fullinfo() with the PCRE_INFO_JIT option. A
! 6594: result of 1 means that JIT compilation was successful. A result of 0
! 6595: means that JIT support is not available, or the pattern was not studied
! 6596: with PCRE_STUDY_JIT_COMPILE, or the JIT compiler was not able to handle
! 6597: the pattern.
! 6598:
! 6599: Once a pattern has been studied, with or without JIT, it can be used as
! 6600: many times as you like for matching different subject strings.
! 6601:
! 6602:
! 6603: UNSUPPORTED OPTIONS AND PATTERN ITEMS
! 6604:
! 6605: The only pcre_exec() options that are supported for JIT execution are
! 6606: PCRE_NO_UTF8_CHECK, PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and
! 6607: PCRE_NOTEMPTY_ATSTART. Note in particular that partial matching is not
! 6608: supported.
! 6609:
! 6610: The unsupported pattern items are:
! 6611:
! 6612: \C match a single byte; not supported in UTF-8 mode
! 6613: (?Cn) callouts
! 6614: (*COMMIT) )
! 6615: (*MARK) )
! 6616: (*PRUNE) ) the backtracking control verbs
! 6617: (*SKIP) )
! 6618: (*THEN) )
! 6619:
! 6620: Support for some of these may be added in future.
! 6621:
! 6622:
! 6623: RETURN VALUES FROM JIT EXECUTION
! 6624:
! 6625: When a pattern is matched using JIT execution, the return values are
! 6626: the same as those given by the interpretive pcre_exec() code, with the
! 6627: addition of one new error code: PCRE_ERROR_JIT_STACKLIMIT. This means
! 6628: that the memory used for the JIT stack was insufficient. See "Control-
! 6629: ling the JIT stack" below for a discussion of JIT stack usage. For com-
! 6630: patibility with the interpretive pcre_exec() code, no more than two-
! 6631: thirds of the ovector argument is used for passing back captured sub-
! 6632: strings.
! 6633:
! 6634: The error code PCRE_ERROR_MATCHLIMIT is returned by the JIT code if
! 6635: searching a very large pattern tree goes on for too long, as it is in
! 6636: the same circumstance when JIT is not used, but the details of exactly
! 6637: what is counted are not the same. The PCRE_ERROR_RECURSIONLIMIT error
! 6638: code is never returned by JIT execution.
! 6639:
! 6640:
! 6641: SAVING AND RESTORING COMPILED PATTERNS
! 6642:
! 6643: The code that is generated by the JIT compiler is architecture-spe-
! 6644: cific, and is also position dependent. For those reasons it cannot be
! 6645: saved (in a file or database) and restored later like the bytecode and
! 6646: other data of a compiled pattern. Saving and restoring compiled pat-
! 6647: terns is not something many people do. More detail about this facility
! 6648: is given in the pcreprecompile documentation. It should be possible to
! 6649: run pcre_study() on a saved and restored pattern, and thereby recreate
! 6650: the JIT data, but because JIT compilation uses significant resources,
! 6651: it is probably not worth doing this; you might as well recompile the
! 6652: original pattern.
! 6653:
! 6654:
! 6655: CONTROLLING THE JIT STACK
! 6656:
! 6657: When the compiled JIT code runs, it needs a block of memory to use as a
! 6658: stack. By default, it uses 32K on the machine stack. However, some
! 6659: large or complicated patterns need more than this. The error
! 6660: PCRE_ERROR_JIT_STACKLIMIT is given when there is not enough stack.
! 6661: Three functions are provided for managing blocks of memory for use as
! 6662: JIT stacks. There is further discussion about the use of JIT stacks in
! 6663: the section entitled "JIT stack FAQ" below.
! 6664:
! 6665: The pcre_jit_stack_alloc() function creates a JIT stack. Its arguments
! 6666: are a starting size and a maximum size, and it returns a pointer to an
! 6667: opaque structure of type pcre_jit_stack, or NULL if there is an error.
! 6668: The pcre_jit_stack_free() function can be used to free a stack that is
! 6669: no longer needed. (For the technically minded: the address space is
! 6670: allocated by mmap or VirtualAlloc.)
! 6671:
! 6672: JIT uses far less memory for recursion than the interpretive code, and
! 6673: a maximum stack size of 512K to 1M should be more than enough for any
! 6674: pattern.
! 6675:
! 6676: The pcre_assign_jit_stack() function specifies which stack JIT code
! 6677: should use. Its arguments are as follows:
! 6678:
! 6679: pcre_extra *extra
! 6680: pcre_jit_callback callback
! 6681: void *data
! 6682:
! 6683: The extra argument must be the result of studying a pattern with
! 6684: PCRE_STUDY_JIT_COMPILE. There are three cases for the values of the
! 6685: other two options:
! 6686:
! 6687: (1) If callback is NULL and data is NULL, an internal 32K block
! 6688: on the machine stack is used.
! 6689:
! 6690: (2) If callback is NULL and data is not NULL, data must be
! 6691: a valid JIT stack, the result of calling pcre_jit_stack_alloc().
! 6692:
! 6693: (3) If callback not NULL, it must point to a function that is called
! 6694: with data as an argument at the start of matching, in order to
! 6695: set up a JIT stack. If the result is NULL, the internal 32K stack
! 6696: is used; otherwise the return value must be a valid JIT stack,
! 6697: the result of calling pcre_jit_stack_alloc().
! 6698:
! 6699: You may safely assign the same JIT stack to more than one pattern, as
! 6700: long as they are all matched sequentially in the same thread. In a mul-
! 6701: tithread application, each thread must use its own JIT stack.
! 6702:
! 6703: Strictly speaking, even more is allowed. You can assign the same stack
! 6704: to any number of patterns as long as they are not used for matching by
! 6705: multiple threads at the same time. For example, you can assign the same
! 6706: stack to all compiled patterns, and use a global mutex in the callback
! 6707: to wait until the stack is available for use. However, this is an inef-
! 6708: ficient solution, and not recommended.
! 6709:
! 6710: This is a suggestion for how a typical multithreaded program might
! 6711: operate:
! 6712:
! 6713: During thread initalization
! 6714: thread_local_var = pcre_jit_stack_alloc(...)
! 6715:
! 6716: During thread exit
! 6717: pcre_jit_stack_free(thread_local_var)
! 6718:
! 6719: Use a one-line callback function
! 6720: return thread_local_var
! 6721:
! 6722: All the functions described in this section do nothing if JIT is not
! 6723: available, and pcre_assign_jit_stack() does nothing unless the extra
! 6724: argument is non-NULL and points to a pcre_extra block that is the
! 6725: result of a successful study with PCRE_STUDY_JIT_COMPILE.
! 6726:
! 6727:
! 6728: JIT STACK FAQ
! 6729:
! 6730: (1) Why do we need JIT stacks?
! 6731:
! 6732: PCRE (and JIT) is a recursive, depth-first engine, so it needs a stack
! 6733: where the local data of the current node is pushed before checking its
! 6734: child nodes. Allocating real machine stack on some platforms is diffi-
! 6735: cult. For example, the stack chain needs to be updated every time if we
! 6736: extend the stack on PowerPC. Although it is possible, its updating
! 6737: time overhead decreases performance. So we do the recursion in memory.
! 6738:
! 6739: (2) Why don't we simply allocate blocks of memory with malloc()?
! 6740:
! 6741: Modern operating systems have a nice feature: they can reserve an
! 6742: address space instead of allocating memory. We can safely allocate mem-
! 6743: ory pages inside this address space, so the stack could grow without
! 6744: moving memory data (this is important because of pointers). Thus we can
! 6745: allocate 1M address space, and use only a single memory page (usually
! 6746: 4K) if that is enough. However, we can still grow up to 1M anytime if
! 6747: needed.
! 6748:
! 6749: (3) Who "owns" a JIT stack?
! 6750:
! 6751: The owner of the stack is the user program, not the JIT studied pattern
! 6752: or anything else. The user program must ensure that if a stack is used
! 6753: by pcre_exec(), (that is, it is assigned to the pattern currently run-
! 6754: ning), that stack must not be used by any other threads (to avoid over-
! 6755: writing the same memory area). The best practice for multithreaded pro-
! 6756: grams is to allocate a stack for each thread, and return this stack
! 6757: through the JIT callback function.
! 6758:
! 6759: (4) When should a JIT stack be freed?
! 6760:
! 6761: You can free a JIT stack at any time, as long as it will not be used by
! 6762: pcre_exec() again. When you assign the stack to a pattern, only a
! 6763: pointer is set. There is no reference counting or any other magic. You
! 6764: can free the patterns and stacks in any order, anytime. Just do not
! 6765: call pcre_exec() with a pattern pointing to an already freed stack, as
! 6766: that will cause SEGFAULT. (Also, do not free a stack currently used by
! 6767: pcre_exec() in another thread). You can also replace the stack for a
! 6768: pattern at any time. You can even free the previous stack before
! 6769: assigning a replacement.
! 6770:
! 6771: (5) Should I allocate/free a stack every time before/after calling
! 6772: pcre_exec()?
! 6773:
! 6774: No, because this is too costly in terms of resources. However, you
! 6775: could implement some clever idea which release the stack if it is not
! 6776: used in let's say two minutes. The JIT callback can help to achive this
! 6777: without keeping a list of the currently JIT studied patterns.
! 6778:
! 6779: (6) OK, the stack is for long term memory allocation. But what happens
! 6780: if a pattern causes stack overflow with a stack of 1M? Is that 1M kept
! 6781: until the stack is freed?
! 6782:
! 6783: Especially on embedded sytems, it might be a good idea to release mem-
! 6784: ory sometimes without freeing the stack. There is no API for this at
! 6785: the moment. Probably a function call which returns with the currently
! 6786: allocated memory for any stack and another which allows releasing mem-
! 6787: ory (shrinking the stack) would be a good idea if someone needs this.
! 6788:
! 6789: (7) This is too much of a headache. Isn't there any better solution for
! 6790: JIT stack handling?
! 6791:
! 6792: No, thanks to Windows. If POSIX threads were used everywhere, we could
! 6793: throw out this complicated API.
! 6794:
! 6795:
! 6796: EXAMPLE CODE
! 6797:
! 6798: This is a single-threaded example that specifies a JIT stack without
! 6799: using a callback.
! 6800:
! 6801: int rc;
! 6802: int ovector[30];
! 6803: pcre *re;
! 6804: pcre_extra *extra;
! 6805: pcre_jit_stack *jit_stack;
! 6806:
! 6807: re = pcre_compile(pattern, 0, &error, &erroffset, NULL);
! 6808: /* Check for errors */
! 6809: extra = pcre_study(re, PCRE_STUDY_JIT_COMPILE, &error);
! 6810: jit_stack = pcre_jit_stack_alloc(32*1024, 512*1024);
! 6811: /* Check for error (NULL) */
! 6812: pcre_assign_jit_stack(extra, NULL, jit_stack);
! 6813: rc = pcre_exec(re, extra, subject, length, 0, 0, ovector, 30);
! 6814: /* Check results */
! 6815: pcre_free(re);
! 6816: pcre_free_study(extra);
! 6817: pcre_jit_stack_free(jit_stack);
! 6818:
! 6819:
! 6820: SEE ALSO
! 6821:
! 6822: pcreapi(3)
! 6823:
! 6824:
! 6825: AUTHOR
! 6826:
! 6827: Philip Hazel (FAQ by Zoltan Herczeg)
! 6828: University Computing Service
! 6829: Cambridge CB2 3QH, England.
! 6830:
! 6831:
! 6832: REVISION
! 6833:
! 6834: Last updated: 26 November 2011
! 6835: Copyright (c) 1997-2011 University of Cambridge.
! 6836: ------------------------------------------------------------------------------
! 6837:
! 6838:
! 6839: PCREPARTIAL(3) PCREPARTIAL(3)
! 6840:
! 6841:
! 6842: NAME
! 6843: PCRE - Perl-compatible regular expressions
! 6844:
! 6845:
! 6846: PARTIAL MATCHING IN PCRE
! 6847:
! 6848: In normal use of PCRE, if the subject string that is passed to
! 6849: pcre_exec() or pcre_dfa_exec() matches as far as it goes, but is too
! 6850: short to match the entire pattern, PCRE_ERROR_NOMATCH is returned.
! 6851: There are circumstances where it might be helpful to distinguish this
! 6852: case from other cases in which there is no match.
! 6853:
! 6854: Consider, for example, an application where a human is required to type
! 6855: in data for a field with specific formatting requirements. An example
! 6856: might be a date in the form ddmmmyy, defined by this pattern:
! 6857:
! 6858: ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$
! 6859:
! 6860: If the application sees the user's keystrokes one by one, and can check
! 6861: that what has been typed so far is potentially valid, it is able to
! 6862: raise an error as soon as a mistake is made, by beeping and not
! 6863: reflecting the character that has been typed, for example. This immedi-
! 6864: ate feedback is likely to be a better user interface than a check that
! 6865: is delayed until the entire string has been entered. Partial matching
! 6866: can also be useful when the subject string is very long and is not all
! 6867: available at once.
! 6868:
! 6869: PCRE supports partial matching by means of the PCRE_PARTIAL_SOFT and
! 6870: PCRE_PARTIAL_HARD options, which can be set when calling pcre_exec() or
! 6871: pcre_dfa_exec(). For backwards compatibility, PCRE_PARTIAL is a synonym
! 6872: for PCRE_PARTIAL_SOFT. The essential difference between the two options
! 6873: is whether or not a partial match is preferred to an alternative com-
! 6874: plete match, though the details differ between the two matching func-
! 6875: tions. If both options are set, PCRE_PARTIAL_HARD takes precedence.
! 6876:
! 6877: Setting a partial matching option for pcre_exec() disables the use of
! 6878: any just-in-time code that was set up by calling pcre_study() with the
! 6879: PCRE_STUDY_JIT_COMPILE option. It also disables two of PCRE's standard
! 6880: optimizations. PCRE remembers the last literal byte in a pattern, and
! 6881: abandons matching immediately if such a byte is not present in the sub-
! 6882: ject string. This optimization cannot be used for a subject string that
! 6883: might match only partially. If the pattern was studied, PCRE knows the
! 6884: minimum length of a matching string, and does not bother to run the
! 6885: matching function on shorter strings. This optimization is also dis-
! 6886: abled for partial matching.
! 6887:
! 6888:
! 6889: PARTIAL MATCHING USING pcre_exec()
! 6890:
! 6891: A partial match occurs during a call to pcre_exec() when the end of the
! 6892: subject string is reached successfully, but matching cannot continue
! 6893: because more characters are needed. However, at least one character in
! 6894: the subject must have been inspected. This character need not form part
! 6895: of the final matched string; lookbehind assertions and the \K escape
! 6896: sequence provide ways of inspecting characters before the start of a
! 6897: matched substring. The requirement for inspecting at least one charac-
! 6898: ter exists because an empty string can always be matched; without such
! 6899: a restriction there would always be a partial match of an empty string
! 6900: at the end of the subject.
! 6901:
! 6902: If there are at least two slots in the offsets vector when pcre_exec()
! 6903: returns with a partial match, the first slot is set to the offset of
! 6904: the earliest character that was inspected when the partial match was
! 6905: found. For convenience, the second offset points to the end of the sub-
! 6906: ject so that a substring can easily be identified.
! 6907:
! 6908: For the majority of patterns, the first offset identifies the start of
! 6909: the partially matched string. However, for patterns that contain look-
! 6910: behind assertions, or \K, or begin with \b or \B, earlier characters
! 6911: have been inspected while carrying out the match. For example:
! 6912:
! 6913: /(?<=abc)123/
! 6914:
! 6915: This pattern matches "123", but only if it is preceded by "abc". If the
! 6916: subject string is "xyzabc12", the offsets after a partial match are for
! 6917: the substring "abc12", because all these characters are needed if
! 6918: another match is tried with extra characters added to the subject.
! 6919:
! 6920: What happens when a partial match is identified depends on which of the
! 6921: two partial matching options are set.
! 6922:
! 6923: PCRE_PARTIAL_SOFT with pcre_exec()
! 6924:
! 6925: If PCRE_PARTIAL_SOFT is set when pcre_exec() identifies a partial
! 6926: match, the partial match is remembered, but matching continues as nor-
! 6927: mal, and other alternatives in the pattern are tried. If no complete
! 6928: match can be found, pcre_exec() returns PCRE_ERROR_PARTIAL instead of
! 6929: PCRE_ERROR_NOMATCH.
! 6930:
! 6931: This option is "soft" because it prefers a complete match over a par-
! 6932: tial match. All the various matching items in a pattern behave as if
! 6933: the subject string is potentially complete. For example, \z, \Z, and $
! 6934: match at the end of the subject, as normal, and for \b and \B the end
! 6935: of the subject is treated as a non-alphanumeric.
! 6936:
! 6937: If there is more than one partial match, the first one that was found
! 6938: provides the data that is returned. Consider this pattern:
! 6939:
! 6940: /123\w+X|dogY/
! 6941:
! 6942: If this is matched against the subject string "abc123dog", both alter-
! 6943: natives fail to match, but the end of the subject is reached during
! 6944: matching, so PCRE_ERROR_PARTIAL is returned. The offsets are set to 3
! 6945: and 9, identifying "123dog" as the first partial match that was found.
! 6946: (In this example, there are two partial matches, because "dog" on its
! 6947: own partially matches the second alternative.)
! 6948:
! 6949: PCRE_PARTIAL_HARD with pcre_exec()
! 6950:
! 6951: If PCRE_PARTIAL_HARD is set for pcre_exec(), it returns PCRE_ERROR_PAR-
! 6952: TIAL as soon as a partial match is found, without continuing to search
! 6953: for possible complete matches. This option is "hard" because it prefers
! 6954: an earlier partial match over a later complete match. For this reason,
! 6955: the assumption is made that the end of the supplied subject string may
! 6956: not be the true end of the available data, and so, if \z, \Z, \b, \B,
! 6957: or $ are encountered at the end of the subject, the result is
! 6958: PCRE_ERROR_PARTIAL.
! 6959:
! 6960: Setting PCRE_PARTIAL_HARD also affects the way pcre_exec() checks UTF-8
! 6961: subject strings for validity. Normally, an invalid UTF-8 sequence
! 6962: causes the error PCRE_ERROR_BADUTF8. However, in the special case of a
! 6963: truncated UTF-8 character at the end of the subject, PCRE_ERROR_SHORT-
! 6964: UTF8 is returned when PCRE_PARTIAL_HARD is set.
! 6965:
! 6966: Comparing hard and soft partial matching
! 6967:
! 6968: The difference between the two partial matching options can be illus-
! 6969: trated by a pattern such as:
! 6970:
! 6971: /dog(sbody)?/
! 6972:
! 6973: This matches either "dog" or "dogsbody", greedily (that is, it prefers
! 6974: the longer string if possible). If it is matched against the string
! 6975: "dog" with PCRE_PARTIAL_SOFT, it yields a complete match for "dog".
! 6976: However, if PCRE_PARTIAL_HARD is set, the result is PCRE_ERROR_PARTIAL.
! 6977: On the other hand, if the pattern is made ungreedy the result is dif-
! 6978: ferent:
! 6979:
! 6980: /dog(sbody)??/
! 6981:
! 6982: In this case the result is always a complete match because pcre_exec()
! 6983: finds that first, and it never continues after finding a match. It
! 6984: might be easier to follow this explanation by thinking of the two pat-
! 6985: terns like this:
! 6986:
! 6987: /dog(sbody)?/ is the same as /dogsbody|dog/
! 6988: /dog(sbody)??/ is the same as /dog|dogsbody/
! 6989:
! 6990: The second pattern will never match "dogsbody" when pcre_exec() is
! 6991: used, because it will always find the shorter match first.
! 6992:
! 6993:
! 6994: PARTIAL MATCHING USING pcre_dfa_exec()
! 6995:
! 6996: The pcre_dfa_exec() function moves along the subject string character
! 6997: by character, without backtracking, searching for all possible matches
! 6998: simultaneously. If the end of the subject is reached before the end of
! 6999: the pattern, there is the possibility of a partial match, again pro-
! 7000: vided that at least one character has been inspected.
! 7001:
! 7002: When PCRE_PARTIAL_SOFT is set, PCRE_ERROR_PARTIAL is returned only if
! 7003: there have been no complete matches. Otherwise, the complete matches
! 7004: are returned. However, if PCRE_PARTIAL_HARD is set, a partial match
! 7005: takes precedence over any complete matches. The portion of the string
! 7006: that was inspected when the longest partial match was found is set as
! 7007: the first matching string, provided there are at least two slots in the
! 7008: offsets vector.
! 7009:
! 7010: Because pcre_dfa_exec() always searches for all possible matches, and
! 7011: there is no difference between greedy and ungreedy repetition, its be-
! 7012: haviour is different from pcre_exec when PCRE_PARTIAL_HARD is set. Con-
! 7013: sider the string "dog" matched against the ungreedy pattern shown
! 7014: above:
! 7015:
! 7016: /dog(sbody)??/
! 7017:
! 7018: Whereas pcre_exec() stops as soon as it finds the complete match for
! 7019: "dog", pcre_dfa_exec() also finds the partial match for "dogsbody", and
! 7020: so returns that when PCRE_PARTIAL_HARD is set.
! 7021:
! 7022:
! 7023: PARTIAL MATCHING AND WORD BOUNDARIES
! 7024:
! 7025: If a pattern ends with one of sequences \b or \B, which test for word
! 7026: boundaries, partial matching with PCRE_PARTIAL_SOFT can give counter-
! 7027: intuitive results. Consider this pattern:
! 7028:
! 7029: /\bcat\b/
! 7030:
! 7031: This matches "cat", provided there is a word boundary at either end. If
! 7032: the subject string is "the cat", the comparison of the final "t" with a
! 7033: following character cannot take place, so a partial match is found.
! 7034: However, pcre_exec() carries on with normal matching, which matches \b
! 7035: at the end of the subject when the last character is a letter, thus
! 7036: finding a complete match. The result, therefore, is not PCRE_ERROR_PAR-
! 7037: TIAL. The same thing happens with pcre_dfa_exec(), because it also
! 7038: finds the complete match.
! 7039:
! 7040: Using PCRE_PARTIAL_HARD in this case does yield PCRE_ERROR_PARTIAL,
! 7041: because then the partial match takes precedence.
! 7042:
! 7043:
! 7044: FORMERLY RESTRICTED PATTERNS
! 7045:
! 7046: For releases of PCRE prior to 8.00, because of the way certain internal
! 7047: optimizations were implemented in the pcre_exec() function, the
! 7048: PCRE_PARTIAL option (predecessor of PCRE_PARTIAL_SOFT) could not be
! 7049: used with all patterns. From release 8.00 onwards, the restrictions no
! 7050: longer apply, and partial matching with pcre_exec() can be requested
! 7051: for any pattern.
! 7052:
! 7053: Items that were formerly restricted were repeated single characters and
! 7054: repeated metasequences. If PCRE_PARTIAL was set for a pattern that did
! 7055: not conform to the restrictions, pcre_exec() returned the error code
! 7056: PCRE_ERROR_BADPARTIAL (-13). This error code is no longer in use. The
! 7057: PCRE_INFO_OKPARTIAL call to pcre_fullinfo() to find out if a compiled
! 7058: pattern can be used for partial matching now always returns 1.
! 7059:
! 7060:
! 7061: EXAMPLE OF PARTIAL MATCHING USING PCRETEST
! 7062:
! 7063: If the escape sequence \P is present in a pcretest data line, the
! 7064: PCRE_PARTIAL_SOFT option is used for the match. Here is a run of
! 7065: pcretest that uses the date example quoted above:
! 7066:
! 7067: re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
! 7068: data> 25jun04\P
! 7069: 0: 25jun04
! 7070: 1: jun
! 7071: data> 25dec3\P
! 7072: Partial match: 23dec3
! 7073: data> 3ju\P
! 7074: Partial match: 3ju
! 7075: data> 3juj\P
! 7076: No match
! 7077: data> j\P
! 7078: No match
! 7079:
! 7080: The first data string is matched completely, so pcretest shows the
! 7081: matched substrings. The remaining four strings do not match the com-
! 7082: plete pattern, but the first two are partial matches. Similar output is
! 7083: obtained when pcre_dfa_exec() is used.
! 7084:
! 7085: If the escape sequence \P is present more than once in a pcretest data
! 7086: line, the PCRE_PARTIAL_HARD option is set for the match.
! 7087:
! 7088:
! 7089: MULTI-SEGMENT MATCHING WITH pcre_dfa_exec()
! 7090:
! 7091: When a partial match has been found using pcre_dfa_exec(), it is possi-
! 7092: ble to continue the match by providing additional subject data and
! 7093: calling pcre_dfa_exec() again with the same compiled regular expres-
! 7094: sion, this time setting the PCRE_DFA_RESTART option. You must pass the
! 7095: same working space as before, because this is where details of the pre-
! 7096: vious partial match are stored. Here is an example using pcretest,
! 7097: using the \R escape sequence to set the PCRE_DFA_RESTART option (\D
! 7098: specifies the use of pcre_dfa_exec()):
! 7099:
! 7100: re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
! 7101: data> 23ja\P\D
! 7102: Partial match: 23ja
! 7103: data> n05\R\D
! 7104: 0: n05
! 7105:
! 7106: The first call has "23ja" as the subject, and requests partial match-
! 7107: ing; the second call has "n05" as the subject for the continued
! 7108: (restarted) match. Notice that when the match is complete, only the
! 7109: last part is shown; PCRE does not retain the previously partially-
! 7110: matched string. It is up to the calling program to do that if it needs
! 7111: to.
! 7112:
! 7113: You can set the PCRE_PARTIAL_SOFT or PCRE_PARTIAL_HARD options with
! 7114: PCRE_DFA_RESTART to continue partial matching over multiple segments.
! 7115: This facility can be used to pass very long subject strings to
! 7116: pcre_dfa_exec().
! 7117:
! 7118:
! 7119: MULTI-SEGMENT MATCHING WITH pcre_exec()
! 7120:
! 7121: From release 8.00, pcre_exec() can also be used to do multi-segment
! 7122: matching. Unlike pcre_dfa_exec(), it is not possible to restart the
! 7123: previous match with a new segment of data. Instead, new data must be
! 7124: added to the previous subject string, and the entire match re-run,
! 7125: starting from the point where the partial match occurred. Earlier data
! 7126: can be discarded. It is best to use PCRE_PARTIAL_HARD in this situa-
! 7127: tion, because it does not treat the end of a segment as the end of the
! 7128: subject when matching \z, \Z, \b, \B, and $. Consider an unanchored
! 7129: pattern that matches dates:
! 7130:
! 7131: re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
! 7132: data> The date is 23ja\P\P
! 7133: Partial match: 23ja
! 7134:
! 7135: At this stage, an application could discard the text preceding "23ja",
! 7136: add on text from the next segment, and call pcre_exec() again. Unlike
! 7137: pcre_dfa_exec(), the entire matching string must always be available,
! 7138: and the complete matching process occurs for each call, so more memory
! 7139: and more processing time is needed.
! 7140:
! 7141: Note: If the pattern contains lookbehind assertions, or \K, or starts
! 7142: with \b or \B, the string that is returned for a partial match will
! 7143: include characters that precede the partially matched string itself,
! 7144: because these must be retained when adding on more characters for a
! 7145: subsequent matching attempt.
! 7146:
! 7147:
! 7148: ISSUES WITH MULTI-SEGMENT MATCHING
! 7149:
! 7150: Certain types of pattern may give problems with multi-segment matching,
! 7151: whichever matching function is used.
! 7152:
! 7153: 1. If the pattern contains a test for the beginning of a line, you need
! 7154: to pass the PCRE_NOTBOL option when the subject string for any call
! 7155: does start at the beginning of a line. There is also a PCRE_NOTEOL
! 7156: option, but in practice when doing multi-segment matching you should be
! 7157: using PCRE_PARTIAL_HARD, which includes the effect of PCRE_NOTEOL.
! 7158:
! 7159: 2. Lookbehind assertions at the start of a pattern are catered for in
! 7160: the offsets that are returned for a partial match. However, in theory,
! 7161: a lookbehind assertion later in the pattern could require even earlier
! 7162: characters to be inspected, and it might not have been reached when a
! 7163: partial match occurs. This is probably an extremely unlikely case; you
! 7164: could guard against it to a certain extent by always including extra
! 7165: characters at the start.
! 7166:
! 7167: 3. Matching a subject string that is split into multiple segments may
! 7168: not always produce exactly the same result as matching over one single
! 7169: long string, especially when PCRE_PARTIAL_SOFT is used. The section
! 7170: "Partial Matching and Word Boundaries" above describes an issue that
! 7171: arises if the pattern ends with \b or \B. Another kind of difference
! 7172: may occur when there are multiple matching possibilities, because (for
! 7173: PCRE_PARTIAL_SOFT) a partial match result is given only when there are
! 7174: no completed matches. This means that as soon as the shortest match has
! 7175: been found, continuation to a new subject segment is no longer possi-
! 7176: ble. Consider again this pcretest example:
! 7177:
! 7178: re> /dog(sbody)?/
! 7179: data> dogsb\P
! 7180: 0: dog
! 7181: data> do\P\D
! 7182: Partial match: do
! 7183: data> gsb\R\P\D
! 7184: 0: g
! 7185: data> dogsbody\D
! 7186: 0: dogsbody
! 7187: 1: dog
! 7188:
! 7189: The first data line passes the string "dogsb" to pcre_exec(), setting
! 7190: the PCRE_PARTIAL_SOFT option. Although the string is a partial match
! 7191: for "dogsbody", the result is not PCRE_ERROR_PARTIAL, because the
! 7192: shorter string "dog" is a complete match. Similarly, when the subject
! 7193: is presented to pcre_dfa_exec() in several parts ("do" and "gsb" being
! 7194: the first two) the match stops when "dog" has been found, and it is not
! 7195: possible to continue. On the other hand, if "dogsbody" is presented as
! 7196: a single string, pcre_dfa_exec() finds both matches.
! 7197:
! 7198: Because of these problems, it is best to use PCRE_PARTIAL_HARD when
! 7199: matching multi-segment data. The example above then behaves differ-
! 7200: ently:
! 7201:
! 7202: re> /dog(sbody)?/
! 7203: data> dogsb\P\P
! 7204: Partial match: dogsb
! 7205: data> do\P\D
! 7206: Partial match: do
! 7207: data> gsb\R\P\P\D
! 7208: Partial match: gsb
! 7209:
! 7210: 4. Patterns that contain alternatives at the top level which do not all
! 7211: start with the same pattern item may not work as expected when
! 7212: PCRE_DFA_RESTART is used with pcre_dfa_exec(). For example, consider
! 7213: this pattern:
! 7214:
! 7215: 1234|3789
! 7216:
! 7217: If the first part of the subject is "ABC123", a partial match of the
! 7218: first alternative is found at offset 3. There is no partial match for
! 7219: the second alternative, because such a match does not start at the same
! 7220: point in the subject string. Attempting to continue with the string
! 7221: "7890" does not yield a match because only those alternatives that
! 7222: match at one point in the subject are remembered. The problem arises
! 7223: because the start of the second alternative matches within the first
! 7224: alternative. There is no problem with anchored patterns or patterns
! 7225: such as:
! 7226:
! 7227: 1234|ABCD
! 7228:
! 7229: where no string can be a partial match for both alternatives. This is
! 7230: not a problem if pcre_exec() is used, because the entire match has to
! 7231: be rerun each time:
! 7232:
! 7233: re> /1234|3789/
! 7234: data> ABC123\P\P
! 7235: Partial match: 123
! 7236: data> 1237890
! 7237: 0: 3789
! 7238:
! 7239: Of course, instead of using PCRE_DFA_RESTART, the same technique of re-
! 7240: running the entire match can also be used with pcre_dfa_exec(). Another
! 7241: possibility is to work with two buffers. If a partial match at offset n
! 7242: in the first buffer is followed by "no match" when PCRE_DFA_RESTART is
! 7243: used on the second buffer, you can then try a new match starting at
! 7244: offset n+1 in the first buffer.
! 7245:
! 7246:
! 7247: AUTHOR
! 7248:
! 7249: Philip Hazel
! 7250: University Computing Service
! 7251: Cambridge CB2 3QH, England.
! 7252:
! 7253:
! 7254: REVISION
! 7255:
! 7256: Last updated: 26 August 2011
! 7257: Copyright (c) 1997-2011 University of Cambridge.
! 7258: ------------------------------------------------------------------------------
! 7259:
! 7260:
! 7261: PCREPRECOMPILE(3) PCREPRECOMPILE(3)
! 7262:
! 7263:
! 7264: NAME
! 7265: PCRE - Perl-compatible regular expressions
! 7266:
! 7267:
! 7268: SAVING AND RE-USING PRECOMPILED PCRE PATTERNS
! 7269:
! 7270: If you are running an application that uses a large number of regular
! 7271: expression patterns, it may be useful to store them in a precompiled
! 7272: form instead of having to compile them every time the application is
! 7273: run. If you are not using any private character tables (see the
! 7274: pcre_maketables() documentation), this is relatively straightforward.
! 7275: If you are using private tables, it is a little bit more complicated.
! 7276: However, if you are using the just-in-time optimization feature of
! 7277: pcre_study(), it is not possible to save and reload the JIT data.
! 7278:
! 7279: If you save compiled patterns to a file, you can copy them to a differ-
! 7280: ent host and run them there. This works even if the new host has the
! 7281: opposite endianness to the one on which the patterns were compiled.
! 7282: There may be a small performance penalty, but it should be insignifi-
! 7283: cant. However, compiling regular expressions with one version of PCRE
! 7284: for use with a different version is not guaranteed to work and may
! 7285: cause crashes, and saving and restoring a compiled pattern loses any
! 7286: JIT optimization data.
! 7287:
! 7288:
! 7289: SAVING A COMPILED PATTERN
! 7290:
! 7291: The value returned by pcre_compile() points to a single block of memory
! 7292: that holds the compiled pattern and associated data. You can find the
! 7293: length of this block in bytes by calling pcre_fullinfo() with an argu-
! 7294: ment of PCRE_INFO_SIZE. You can then save the data in any appropriate
! 7295: manner. Here is sample code that compiles a pattern and writes it to a
! 7296: file. It assumes that the variable fd refers to a file that is open for
! 7297: output:
! 7298:
! 7299: int erroroffset, rc, size;
! 7300: char *error;
! 7301: pcre *re;
! 7302:
! 7303: re = pcre_compile("my pattern", 0, &error, &erroroffset, NULL);
! 7304: if (re == NULL) { ... handle errors ... }
! 7305: rc = pcre_fullinfo(re, NULL, PCRE_INFO_SIZE, &size);
! 7306: if (rc < 0) { ... handle errors ... }
! 7307: rc = fwrite(re, 1, size, fd);
! 7308: if (rc != size) { ... handle errors ... }
! 7309:
! 7310: In this example, the bytes that comprise the compiled pattern are
! 7311: copied exactly. Note that this is binary data that may contain any of
! 7312: the 256 possible byte values. On systems that make a distinction
! 7313: between binary and non-binary data, be sure that the file is opened for
! 7314: binary output.
! 7315:
! 7316: If you want to write more than one pattern to a file, you will have to
! 7317: devise a way of separating them. For binary data, preceding each pat-
! 7318: tern with its length is probably the most straightforward approach.
! 7319: Another possibility is to write out the data in hexadecimal instead of
! 7320: binary, one pattern to a line.
! 7321:
! 7322: Saving compiled patterns in a file is only one possible way of storing
! 7323: them for later use. They could equally well be saved in a database, or
! 7324: in the memory of some daemon process that passes them via sockets to
! 7325: the processes that want them.
! 7326:
! 7327: If the pattern has been studied, it is also possible to save the normal
! 7328: study data in a similar way to the compiled pattern itself. However, if
! 7329: the PCRE_STUDY_JIT_COMPILE was used, the just-in-time data that is cre-
! 7330: ated cannot be saved because it is too dependent on the current envi-
! 7331: ronment. When studying generates additional information, pcre_study()
! 7332: returns a pointer to a pcre_extra data block. Its format is defined in
! 7333: the section on matching a pattern in the pcreapi documentation. The
! 7334: study_data field points to the binary study data, and this is what you
! 7335: must save (not the pcre_extra block itself). The length of the study
! 7336: data can be obtained by calling pcre_fullinfo() with an argument of
! 7337: PCRE_INFO_STUDYSIZE. Remember to check that pcre_study() did return a
! 7338: non-NULL value before trying to save the study data.
! 7339:
! 7340:
! 7341: RE-USING A PRECOMPILED PATTERN
! 7342:
! 7343: Re-using a precompiled pattern is straightforward. Having reloaded it
! 7344: into main memory, you pass its pointer to pcre_exec() or
! 7345: pcre_dfa_exec() in the usual way. This should work even on another
! 7346: host, and even if that host has the opposite endianness to the one
! 7347: where the pattern was compiled.
! 7348:
! 7349: However, if you passed a pointer to custom character tables when the
! 7350: pattern was compiled (the tableptr argument of pcre_compile()), you
! 7351: must now pass a similar pointer to pcre_exec() or pcre_dfa_exec(),
! 7352: because the value saved with the compiled pattern will obviously be
! 7353: nonsense. A field in a pcre_extra() block is used to pass this data, as
! 7354: described in the section on matching a pattern in the pcreapi documen-
! 7355: tation.
! 7356:
! 7357: If you did not provide custom character tables when the pattern was
! 7358: compiled, the pointer in the compiled pattern is NULL, which causes
! 7359: pcre_exec() to use PCRE's internal tables. Thus, you do not need to
! 7360: take any special action at run time in this case.
! 7361:
! 7362: If you saved study data with the compiled pattern, you need to create
! 7363: your own pcre_extra data block and set the study_data field to point to
! 7364: the reloaded study data. You must also set the PCRE_EXTRA_STUDY_DATA
! 7365: bit in the flags field to indicate that study data is present. Then
! 7366: pass the pcre_extra block to pcre_exec() or pcre_dfa_exec() in the
! 7367: usual way. If the pattern was studied for just-in-time optimization,
! 7368: that data cannot be saved, and so is lost by a save/restore cycle.
! 7369:
! 7370:
! 7371: COMPATIBILITY WITH DIFFERENT PCRE RELEASES
! 7372:
! 7373: In general, it is safest to recompile all saved patterns when you
! 7374: update to a new PCRE release, though not all updates actually require
! 7375: this.
! 7376:
! 7377:
! 7378: AUTHOR
! 7379:
! 7380: Philip Hazel
! 7381: University Computing Service
! 7382: Cambridge CB2 3QH, England.
! 7383:
! 7384:
! 7385: REVISION
! 7386:
! 7387: Last updated: 26 August 2011
! 7388: Copyright (c) 1997-2011 University of Cambridge.
! 7389: ------------------------------------------------------------------------------
! 7390:
! 7391:
! 7392: PCREPERFORM(3) PCREPERFORM(3)
! 7393:
! 7394:
! 7395: NAME
! 7396: PCRE - Perl-compatible regular expressions
! 7397:
! 7398:
! 7399: PCRE PERFORMANCE
! 7400:
! 7401: Two aspects of performance are discussed below: memory usage and pro-
! 7402: cessing time. The way you express your pattern as a regular expression
! 7403: can affect both of them.
! 7404:
! 7405:
! 7406: COMPILED PATTERN MEMORY USAGE
! 7407:
! 7408: Patterns are compiled by PCRE into a reasonably efficient byte code, so
! 7409: that most simple patterns do not use much memory. However, there is one
! 7410: case where the memory usage of a compiled pattern can be unexpectedly
! 7411: large. If a parenthesized subpattern has a quantifier with a minimum
! 7412: greater than 1 and/or a limited maximum, the whole subpattern is
! 7413: repeated in the compiled code. For example, the pattern
! 7414:
! 7415: (abc|def){2,4}
! 7416:
! 7417: is compiled as if it were
! 7418:
! 7419: (abc|def)(abc|def)((abc|def)(abc|def)?)?
! 7420:
! 7421: (Technical aside: It is done this way so that backtrack points within
! 7422: each of the repetitions can be independently maintained.)
! 7423:
! 7424: For regular expressions whose quantifiers use only small numbers, this
! 7425: is not usually a problem. However, if the numbers are large, and par-
! 7426: ticularly if such repetitions are nested, the memory usage can become
! 7427: an embarrassment. For example, the very simple pattern
! 7428:
! 7429: ((ab){1,1000}c){1,3}
! 7430:
! 7431: uses 51K bytes when compiled. When PCRE is compiled with its default
! 7432: internal pointer size of two bytes, the size limit on a compiled pat-
! 7433: tern is 64K, and this is reached with the above pattern if the outer
! 7434: repetition is increased from 3 to 4. PCRE can be compiled to use larger
! 7435: internal pointers and thus handle larger compiled patterns, but it is
! 7436: better to try to rewrite your pattern to use less memory if you can.
! 7437:
! 7438: One way of reducing the memory usage for such patterns is to make use
! 7439: of PCRE's "subroutine" facility. Re-writing the above pattern as
! 7440:
! 7441: ((ab)(?2){0,999}c)(?1){0,2}
! 7442:
! 7443: reduces the memory requirements to 18K, and indeed it remains under 20K
! 7444: even with the outer repetition increased to 100. However, this pattern
! 7445: is not exactly equivalent, because the "subroutine" calls are treated
! 7446: as atomic groups into which there can be no backtracking if there is a
! 7447: subsequent matching failure. Therefore, PCRE cannot do this kind of
! 7448: rewriting automatically. Furthermore, there is a noticeable loss of
! 7449: speed when executing the modified pattern. Nevertheless, if the atomic
! 7450: grouping is not a problem and the loss of speed is acceptable, this
! 7451: kind of rewriting will allow you to process patterns that PCRE cannot
! 7452: otherwise handle.
! 7453:
! 7454:
! 7455: STACK USAGE AT RUN TIME
! 7456:
! 7457: When pcre_exec() is used for matching, certain kinds of pattern can
! 7458: cause it to use large amounts of the process stack. In some environ-
! 7459: ments the default process stack is quite small, and if it runs out the
! 7460: result is often SIGSEGV. This issue is probably the most frequently
! 7461: raised problem with PCRE. Rewriting your pattern can often help. The
! 7462: pcrestack documentation discusses this issue in detail.
! 7463:
! 7464:
! 7465: PROCESSING TIME
! 7466:
! 7467: Certain items in regular expression patterns are processed more effi-
! 7468: ciently than others. It is more efficient to use a character class like
! 7469: [aeiou] than a set of single-character alternatives such as
! 7470: (a|e|i|o|u). In general, the simplest construction that provides the
! 7471: required behaviour is usually the most efficient. Jeffrey Friedl's book
! 7472: contains a lot of useful general discussion about optimizing regular
! 7473: expressions for efficient performance. This document contains a few
! 7474: observations about PCRE.
! 7475:
! 7476: Using Unicode character properties (the \p, \P, and \X escapes) is
! 7477: slow, because PCRE has to scan a structure that contains data for over
! 7478: fifteen thousand characters whenever it needs a character's property.
! 7479: If you can find an alternative pattern that does not use character
! 7480: properties, it will probably be faster.
! 7481:
! 7482: By default, the escape sequences \b, \d, \s, and \w, and the POSIX
! 7483: character classes such as [:alpha:] do not use Unicode properties,
! 7484: partly for backwards compatibility, and partly for performance reasons.
! 7485: However, you can set PCRE_UCP if you want Unicode character properties
! 7486: to be used. This can double the matching time for items such as \d,
! 7487: when matched with pcre_exec(); the performance loss is less with
! 7488: pcre_dfa_exec(), and in both cases there is not much difference for \b.
! 7489:
! 7490: When a pattern begins with .* not in parentheses, or in parentheses
! 7491: that are not the subject of a backreference, and the PCRE_DOTALL option
! 7492: is set, the pattern is implicitly anchored by PCRE, since it can match
! 7493: only at the start of a subject string. However, if PCRE_DOTALL is not
! 7494: set, PCRE cannot make this optimization, because the . metacharacter
! 7495: does not then match a newline, and if the subject string contains new-
! 7496: lines, the pattern may match from the character immediately following
! 7497: one of them instead of from the very start. For example, the pattern
! 7498:
! 7499: .*second
! 7500:
! 7501: matches the subject "first\nand second" (where \n stands for a newline
! 7502: character), with the match starting at the seventh character. In order
! 7503: to do this, PCRE has to retry the match starting after every newline in
! 7504: the subject.
! 7505:
! 7506: If you are using such a pattern with subject strings that do not con-
! 7507: tain newlines, the best performance is obtained by setting PCRE_DOTALL,
! 7508: or starting the pattern with ^.* or ^.*? to indicate explicit anchor-
! 7509: ing. That saves PCRE from having to scan along the subject looking for
! 7510: a newline to restart at.
! 7511:
! 7512: Beware of patterns that contain nested indefinite repeats. These can
! 7513: take a long time to run when applied to a string that does not match.
! 7514: Consider the pattern fragment
! 7515:
! 7516: ^(a+)*
! 7517:
! 7518: This can match "aaaa" in 16 different ways, and this number increases
! 7519: very rapidly as the string gets longer. (The * repeat can match 0, 1,
! 7520: 2, 3, or 4 times, and for each of those cases other than 0 or 4, the +
! 7521: repeats can match different numbers of times.) When the remainder of
! 7522: the pattern is such that the entire match is going to fail, PCRE has in
! 7523: principle to try every possible variation, and this can take an
! 7524: extremely long time, even for relatively short strings.
! 7525:
! 7526: An optimization catches some of the more simple cases such as
! 7527:
! 7528: (a+)*b
! 7529:
! 7530: where a literal character follows. Before embarking on the standard
! 7531: matching procedure, PCRE checks that there is a "b" later in the sub-
! 7532: ject string, and if there is not, it fails the match immediately. How-
! 7533: ever, when there is no following literal this optimization cannot be
! 7534: used. You can see the difference by comparing the behaviour of
! 7535:
! 7536: (a+)*\d
! 7537:
! 7538: with the pattern above. The former gives a failure almost instantly
! 7539: when applied to a whole line of "a" characters, whereas the latter
! 7540: takes an appreciable time with strings longer than about 20 characters.
! 7541:
! 7542: In many cases, the solution to this kind of performance issue is to use
! 7543: an atomic group or a possessive quantifier.
! 7544:
! 7545:
! 7546: AUTHOR
! 7547:
! 7548: Philip Hazel
! 7549: University Computing Service
! 7550: Cambridge CB2 3QH, England.
! 7551:
! 7552:
! 7553: REVISION
! 7554:
! 7555: Last updated: 16 May 2010
! 7556: Copyright (c) 1997-2010 University of Cambridge.
! 7557: ------------------------------------------------------------------------------
! 7558:
! 7559:
! 7560: PCREPOSIX(3) PCREPOSIX(3)
! 7561:
! 7562:
! 7563: NAME
! 7564: PCRE - Perl-compatible regular expressions.
! 7565:
! 7566:
! 7567: SYNOPSIS OF POSIX API
! 7568:
! 7569: #include <pcreposix.h>
! 7570:
! 7571: int regcomp(regex_t *preg, const char *pattern,
! 7572: int cflags);
! 7573:
! 7574: int regexec(regex_t *preg, const char *string,
! 7575: size_t nmatch, regmatch_t pmatch[], int eflags);
! 7576:
! 7577: size_t regerror(int errcode, const regex_t *preg,
! 7578: char *errbuf, size_t errbuf_size);
! 7579:
! 7580: void regfree(regex_t *preg);
! 7581:
! 7582:
! 7583: DESCRIPTION
! 7584:
! 7585: This set of functions provides a POSIX-style API to the PCRE regular
! 7586: expression package. See the pcreapi documentation for a description of
! 7587: PCRE's native API, which contains much additional functionality.
! 7588:
! 7589: The functions described here are just wrapper functions that ultimately
! 7590: call the PCRE native API. Their prototypes are defined in the
! 7591: pcreposix.h header file, and on Unix systems the library itself is
! 7592: called pcreposix.a, so can be accessed by adding -lpcreposix to the
! 7593: command for linking an application that uses them. Because the POSIX
! 7594: functions call the native ones, it is also necessary to add -lpcre.
! 7595:
! 7596: I have implemented only those POSIX option bits that can be reasonably
! 7597: mapped to PCRE native options. In addition, the option REG_EXTENDED is
! 7598: defined with the value zero. This has no effect, but since programs
! 7599: that are written to the POSIX interface often use it, this makes it
! 7600: easier to slot in PCRE as a replacement library. Other POSIX options
! 7601: are not even defined.
! 7602:
! 7603: There are also some other options that are not defined by POSIX. These
! 7604: have been added at the request of users who want to make use of certain
! 7605: PCRE-specific features via the POSIX calling interface.
! 7606:
! 7607: When PCRE is called via these functions, it is only the API that is
! 7608: POSIX-like in style. The syntax and semantics of the regular expres-
! 7609: sions themselves are still those of Perl, subject to the setting of
! 7610: various PCRE options, as described below. "POSIX-like in style" means
! 7611: that the API approximates to the POSIX definition; it is not fully
! 7612: POSIX-compatible, and in multi-byte encoding domains it is probably
! 7613: even less compatible.
! 7614:
! 7615: The header for these functions is supplied as pcreposix.h to avoid any
! 7616: potential clash with other POSIX libraries. It can, of course, be
! 7617: renamed or aliased as regex.h, which is the "correct" name. It provides
! 7618: two structure types, regex_t for compiled internal forms, and reg-
! 7619: match_t for returning captured substrings. It also defines some con-
! 7620: stants whose names start with "REG_"; these are used for setting
! 7621: options and identifying error codes.
! 7622:
! 7623:
! 7624: COMPILING A PATTERN
! 7625:
! 7626: The function regcomp() is called to compile a pattern into an internal
! 7627: form. The pattern is a C string terminated by a binary zero, and is
! 7628: passed in the argument pattern. The preg argument is a pointer to a
! 7629: regex_t structure that is used as a base for storing information about
! 7630: the compiled regular expression.
! 7631:
! 7632: The argument cflags is either zero, or contains one or more of the bits
! 7633: defined by the following macros:
! 7634:
! 7635: REG_DOTALL
! 7636:
! 7637: The PCRE_DOTALL option is set when the regular expression is passed for
! 7638: compilation to the native function. Note that REG_DOTALL is not part of
! 7639: the POSIX standard.
! 7640:
! 7641: REG_ICASE
! 7642:
! 7643: The PCRE_CASELESS option is set when the regular expression is passed
! 7644: for compilation to the native function.
! 7645:
! 7646: REG_NEWLINE
! 7647:
! 7648: The PCRE_MULTILINE option is set when the regular expression is passed
! 7649: for compilation to the native function. Note that this does not mimic
! 7650: the defined POSIX behaviour for REG_NEWLINE (see the following sec-
! 7651: tion).
! 7652:
! 7653: REG_NOSUB
! 7654:
! 7655: The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is
! 7656: passed for compilation to the native function. In addition, when a pat-
! 7657: tern that is compiled with this flag is passed to regexec() for match-
! 7658: ing, the nmatch and pmatch arguments are ignored, and no captured
! 7659: strings are returned.
! 7660:
! 7661: REG_UCP
! 7662:
! 7663: The PCRE_UCP option is set when the regular expression is passed for
! 7664: compilation to the native function. This causes PCRE to use Unicode
! 7665: properties when matchine \d, \w, etc., instead of just recognizing
! 7666: ASCII values. Note that REG_UTF8 is not part of the POSIX standard.
! 7667:
! 7668: REG_UNGREEDY
! 7669:
! 7670: The PCRE_UNGREEDY option is set when the regular expression is passed
! 7671: for compilation to the native function. Note that REG_UNGREEDY is not
! 7672: part of the POSIX standard.
! 7673:
! 7674: REG_UTF8
! 7675:
! 7676: The PCRE_UTF8 option is set when the regular expression is passed for
! 7677: compilation to the native function. This causes the pattern itself and
! 7678: all data strings used for matching it to be treated as UTF-8 strings.
! 7679: Note that REG_UTF8 is not part of the POSIX standard.
! 7680:
! 7681: In the absence of these flags, no options are passed to the native
! 7682: function. This means the the regex is compiled with PCRE default
! 7683: semantics. In particular, the way it handles newline characters in the
! 7684: subject string is the Perl way, not the POSIX way. Note that setting
! 7685: PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE.
! 7686: It does not affect the way newlines are matched by . (they are not) or
! 7687: by a negative class such as [^a] (they are).
! 7688:
! 7689: The yield of regcomp() is zero on success, and non-zero otherwise. The
! 7690: preg structure is filled in on success, and one member of the structure
! 7691: is public: re_nsub contains the number of capturing subpatterns in the
! 7692: regular expression. Various error codes are defined in the header file.
! 7693:
! 7694: NOTE: If the yield of regcomp() is non-zero, you must not attempt to
! 7695: use the contents of the preg structure. If, for example, you pass it to
! 7696: regexec(), the result is undefined and your program is likely to crash.
! 7697:
! 7698:
! 7699: MATCHING NEWLINE CHARACTERS
! 7700:
! 7701: This area is not simple, because POSIX and Perl take different views of
! 7702: things. It is not possible to get PCRE to obey POSIX semantics, but
! 7703: then PCRE was never intended to be a POSIX engine. The following table
! 7704: lists the different possibilities for matching newline characters in
! 7705: PCRE:
! 7706:
! 7707: Default Change with
! 7708:
! 7709: . matches newline no PCRE_DOTALL
! 7710: newline matches [^a] yes not changeable
! 7711: $ matches \n at end yes PCRE_DOLLARENDONLY
! 7712: $ matches \n in middle no PCRE_MULTILINE
! 7713: ^ matches \n in middle no PCRE_MULTILINE
! 7714:
! 7715: This is the equivalent table for POSIX:
! 7716:
! 7717: Default Change with
! 7718:
! 7719: . matches newline yes REG_NEWLINE
! 7720: newline matches [^a] yes REG_NEWLINE
! 7721: $ matches \n at end no REG_NEWLINE
! 7722: $ matches \n in middle no REG_NEWLINE
! 7723: ^ matches \n in middle no REG_NEWLINE
! 7724:
! 7725: PCRE's behaviour is the same as Perl's, except that there is no equiva-
! 7726: lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is
! 7727: no way to stop newline from matching [^a].
! 7728:
! 7729: The default POSIX newline handling can be obtained by setting
! 7730: PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
! 7731: behave exactly as for the REG_NEWLINE action.
! 7732:
! 7733:
! 7734: MATCHING A PATTERN
! 7735:
! 7736: The function regexec() is called to match a compiled pattern preg
! 7737: against a given string, which is by default terminated by a zero byte
! 7738: (but see REG_STARTEND below), subject to the options in eflags. These
! 7739: can be:
! 7740:
! 7741: REG_NOTBOL
! 7742:
! 7743: The PCRE_NOTBOL option is set when calling the underlying PCRE matching
! 7744: function.
! 7745:
! 7746: REG_NOTEMPTY
! 7747:
! 7748: The PCRE_NOTEMPTY option is set when calling the underlying PCRE match-
! 7749: ing function. Note that REG_NOTEMPTY is not part of the POSIX standard.
! 7750: However, setting this option can give more POSIX-like behaviour in some
! 7751: situations.
! 7752:
! 7753: REG_NOTEOL
! 7754:
! 7755: The PCRE_NOTEOL option is set when calling the underlying PCRE matching
! 7756: function.
! 7757:
! 7758: REG_STARTEND
! 7759:
! 7760: The string is considered to start at string + pmatch[0].rm_so and to
! 7761: have a terminating NUL located at string + pmatch[0].rm_eo (there need
! 7762: not actually be a NUL at that location), regardless of the value of
! 7763: nmatch. This is a BSD extension, compatible with but not specified by
! 7764: IEEE Standard 1003.2 (POSIX.2), and should be used with caution in
! 7765: software intended to be portable to other systems. Note that a non-zero
! 7766: rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location
! 7767: of the string, not how it is matched.
! 7768:
! 7769: If the pattern was compiled with the REG_NOSUB flag, no data about any
! 7770: matched strings is returned. The nmatch and pmatch arguments of
! 7771: regexec() are ignored.
! 7772:
! 7773: If the value of nmatch is zero, or if the value pmatch is NULL, no data
! 7774: about any matched strings is returned.
! 7775:
! 7776: Otherwise,the portion of the string that was matched, and also any cap-
! 7777: tured substrings, are returned via the pmatch argument, which points to
! 7778: an array of nmatch structures of type regmatch_t, containing the mem-
! 7779: bers rm_so and rm_eo. These contain the offset to the first character
! 7780: of each substring and the offset to the first character after the end
! 7781: of each substring, respectively. The 0th element of the vector relates
! 7782: to the entire portion of string that was matched; subsequent elements
! 7783: relate to the capturing subpatterns of the regular expression. Unused
! 7784: entries in the array have both structure members set to -1.
! 7785:
! 7786: A successful match yields a zero return; various error codes are
! 7787: defined in the header file, of which REG_NOMATCH is the "expected"
! 7788: failure code.
! 7789:
! 7790:
! 7791: ERROR MESSAGES
! 7792:
! 7793: The regerror() function maps a non-zero errorcode from either regcomp()
! 7794: or regexec() to a printable message. If preg is not NULL, the error
! 7795: should have arisen from the use of that structure. A message terminated
! 7796: by a binary zero is placed in errbuf. The length of the message,
! 7797: including the zero, is limited to errbuf_size. The yield of the func-
! 7798: tion is the size of buffer needed to hold the whole message.
! 7799:
! 7800:
! 7801: MEMORY USAGE
! 7802:
! 7803: Compiling a regular expression causes memory to be allocated and asso-
! 7804: ciated with the preg structure. The function regfree() frees all such
! 7805: memory, after which preg may no longer be used as a compiled expres-
! 7806: sion.
! 7807:
! 7808:
! 7809: AUTHOR
! 7810:
! 7811: Philip Hazel
! 7812: University Computing Service
! 7813: Cambridge CB2 3QH, England.
! 7814:
! 7815:
! 7816: REVISION
! 7817:
! 7818: Last updated: 16 May 2010
! 7819: Copyright (c) 1997-2010 University of Cambridge.
! 7820: ------------------------------------------------------------------------------
! 7821:
! 7822:
! 7823: PCRECPP(3) PCRECPP(3)
! 7824:
! 7825:
! 7826: NAME
! 7827: PCRE - Perl-compatible regular expressions.
! 7828:
! 7829:
! 7830: SYNOPSIS OF C++ WRAPPER
! 7831:
! 7832: #include <pcrecpp.h>
! 7833:
! 7834:
! 7835: DESCRIPTION
! 7836:
! 7837: The C++ wrapper for PCRE was provided by Google Inc. Some additional
! 7838: functionality was added by Giuseppe Maxia. This brief man page was con-
! 7839: structed from the notes in the pcrecpp.h file, which should be con-
! 7840: sulted for further details.
! 7841:
! 7842:
! 7843: MATCHING INTERFACE
! 7844:
! 7845: The "FullMatch" operation checks that supplied text matches a supplied
! 7846: pattern exactly. If pointer arguments are supplied, it copies matched
! 7847: sub-strings that match sub-patterns into them.
! 7848:
! 7849: Example: successful match
! 7850: pcrecpp::RE re("h.*o");
! 7851: re.FullMatch("hello");
! 7852:
! 7853: Example: unsuccessful match (requires full match):
! 7854: pcrecpp::RE re("e");
! 7855: !re.FullMatch("hello");
! 7856:
! 7857: Example: creating a temporary RE object:
! 7858: pcrecpp::RE("h.*o").FullMatch("hello");
! 7859:
! 7860: You can pass in a "const char*" or a "string" for "text". The examples
! 7861: below tend to use a const char*. You can, as in the different examples
! 7862: above, store the RE object explicitly in a variable or use a temporary
! 7863: RE object. The examples below use one mode or the other arbitrarily.
! 7864: Either could correctly be used for any of these examples.
! 7865:
! 7866: You must supply extra pointer arguments to extract matched subpieces.
! 7867:
! 7868: Example: extracts "ruby" into "s" and 1234 into "i"
! 7869: int i;
! 7870: string s;
! 7871: pcrecpp::RE re("(\\w+):(\\d+)");
! 7872: re.FullMatch("ruby:1234", &s, &i);
! 7873:
! 7874: Example: does not try to extract any extra sub-patterns
! 7875: re.FullMatch("ruby:1234", &s);
! 7876:
! 7877: Example: does not try to extract into NULL
! 7878: re.FullMatch("ruby:1234", NULL, &i);
! 7879:
! 7880: Example: integer overflow causes failure
! 7881: !re.FullMatch("ruby:1234567891234", NULL, &i);
! 7882:
! 7883: Example: fails because there aren't enough sub-patterns:
! 7884: !pcrecpp::RE("\\w+:\\d+").FullMatch("ruby:1234", &s);
! 7885:
! 7886: Example: fails because string cannot be stored in integer
! 7887: !pcrecpp::RE("(.*)").FullMatch("ruby", &i);
! 7888:
! 7889: The provided pointer arguments can be pointers to any scalar numeric
! 7890: type, or one of:
! 7891:
! 7892: string (matched piece is copied to string)
! 7893: StringPiece (StringPiece is mutated to point to matched piece)
! 7894: T (where "bool T::ParseFrom(const char*, int)" exists)
! 7895: NULL (the corresponding matched sub-pattern is not copied)
! 7896:
! 7897: The function returns true iff all of the following conditions are sat-
! 7898: isfied:
! 7899:
! 7900: a. "text" matches "pattern" exactly;
! 7901:
! 7902: b. The number of matched sub-patterns is >= number of supplied
! 7903: pointers;
! 7904:
! 7905: c. The "i"th argument has a suitable type for holding the
! 7906: string captured as the "i"th sub-pattern. If you pass in
! 7907: void * NULL for the "i"th argument, or a non-void * NULL
! 7908: of the correct type, or pass fewer arguments than the
! 7909: number of sub-patterns, "i"th captured sub-pattern is
! 7910: ignored.
! 7911:
! 7912: CAVEAT: An optional sub-pattern that does not exist in the matched
! 7913: string is assigned the empty string. Therefore, the following will
! 7914: return false (because the empty string is not a valid number):
! 7915:
! 7916: int number;
! 7917: pcrecpp::RE::FullMatch("abc", "[a-z]+(\\d+)?", &number);
! 7918:
! 7919: The matching interface supports at most 16 arguments per call. If you
! 7920: need more, consider using the more general interface
! 7921: pcrecpp::RE::DoMatch. See pcrecpp.h for the signature for DoMatch.
! 7922:
! 7923: NOTE: Do not use no_arg, which is used internally to mark the end of a
! 7924: list of optional arguments, as a placeholder for missing arguments, as
! 7925: this can lead to segfaults.
! 7926:
! 7927:
! 7928: QUOTING METACHARACTERS
! 7929:
! 7930: You can use the "QuoteMeta" operation to insert backslashes before all
! 7931: potentially meaningful characters in a string. The returned string,
! 7932: used as a regular expression, will exactly match the original string.
! 7933:
! 7934: Example:
! 7935: string quoted = RE::QuoteMeta(unquoted);
! 7936:
! 7937: Note that it's legal to escape a character even if it has no special
! 7938: meaning in a regular expression -- so this function does that. (This
! 7939: also makes it identical to the perl function of the same name; see
! 7940: "perldoc -f quotemeta".) For example, "1.5-2.0?" becomes
! 7941: "1\.5\-2\.0\?".
! 7942:
! 7943:
! 7944: PARTIAL MATCHES
! 7945:
! 7946: You can use the "PartialMatch" operation when you want the pattern to
! 7947: match any substring of the text.
! 7948:
! 7949: Example: simple search for a string:
! 7950: pcrecpp::RE("ell").PartialMatch("hello");
! 7951:
! 7952: Example: find first number in a string:
! 7953: int number;
! 7954: pcrecpp::RE re("(\\d+)");
! 7955: re.PartialMatch("x*100 + 20", &number);
! 7956: assert(number == 100);
! 7957:
! 7958:
! 7959: UTF-8 AND THE MATCHING INTERFACE
! 7960:
! 7961: By default, pattern and text are plain text, one byte per character.
! 7962: The UTF8 flag, passed to the constructor, causes both pattern and
! 7963: string to be treated as UTF-8 text, still a byte stream but potentially
! 7964: multiple bytes per character. In practice, the text is likelier to be
! 7965: UTF-8 than the pattern, but the match returned may depend on the UTF8
! 7966: flag, so always use it when matching UTF8 text. For example, "." will
! 7967: match one byte normally but with UTF8 set may match up to three bytes
! 7968: of a multi-byte character.
! 7969:
! 7970: Example:
! 7971: pcrecpp::RE_Options options;
! 7972: options.set_utf8();
! 7973: pcrecpp::RE re(utf8_pattern, options);
! 7974: re.FullMatch(utf8_string);
! 7975:
! 7976: Example: using the convenience function UTF8():
! 7977: pcrecpp::RE re(utf8_pattern, pcrecpp::UTF8());
! 7978: re.FullMatch(utf8_string);
! 7979:
! 7980: NOTE: The UTF8 flag is ignored if pcre was not configured with the
! 7981: --enable-utf8 flag.
! 7982:
! 7983:
! 7984: PASSING MODIFIERS TO THE REGULAR EXPRESSION ENGINE
! 7985:
! 7986: PCRE defines some modifiers to change the behavior of the regular
! 7987: expression engine. The C++ wrapper defines an auxiliary class,
! 7988: RE_Options, as a vehicle to pass such modifiers to a RE class. Cur-
! 7989: rently, the following modifiers are supported:
! 7990:
! 7991: modifier description Perl corresponding
! 7992:
! 7993: PCRE_CASELESS case insensitive match /i
! 7994: PCRE_MULTILINE multiple lines match /m
! 7995: PCRE_DOTALL dot matches newlines /s
! 7996: PCRE_DOLLAR_ENDONLY $ matches only at end N/A
! 7997: PCRE_EXTRA strict escape parsing N/A
! 7998: PCRE_EXTENDED ignore whitespaces /x
! 7999: PCRE_UTF8 handles UTF8 chars built-in
! 8000: PCRE_UNGREEDY reverses * and *? N/A
! 8001: PCRE_NO_AUTO_CAPTURE disables capturing parens N/A (*)
! 8002:
! 8003: (*) Both Perl and PCRE allow non capturing parentheses by means of the
! 8004: "?:" modifier within the pattern itself. e.g. (?:ab|cd) does not cap-
! 8005: ture, while (ab|cd) does.
! 8006:
! 8007: For a full account on how each modifier works, please check the PCRE
! 8008: API reference page.
! 8009:
! 8010: For each modifier, there are two member functions whose name is made
! 8011: out of the modifier in lowercase, without the "PCRE_" prefix. For
! 8012: instance, PCRE_CASELESS is handled by
! 8013:
! 8014: bool caseless()
! 8015:
! 8016: which returns true if the modifier is set, and
! 8017:
! 8018: RE_Options & set_caseless(bool)
! 8019:
! 8020: which sets or unsets the modifier. Moreover, PCRE_EXTRA_MATCH_LIMIT can
! 8021: be accessed through the set_match_limit() and match_limit() member
! 8022: functions. Setting match_limit to a non-zero value will limit the exe-
! 8023: cution of pcre to keep it from doing bad things like blowing the stack
! 8024: or taking an eternity to return a result. A value of 5000 is good
! 8025: enough to stop stack blowup in a 2MB thread stack. Setting match_limit
! 8026: to zero disables match limiting. Alternatively, you can call
! 8027: match_limit_recursion() which uses PCRE_EXTRA_MATCH_LIMIT_RECURSION to
! 8028: limit how much PCRE recurses. match_limit() limits the number of
! 8029: matches PCRE does; match_limit_recursion() limits the depth of internal
! 8030: recursion, and therefore the amount of stack that is used.
! 8031:
! 8032: Normally, to pass one or more modifiers to a RE class, you declare a
! 8033: RE_Options object, set the appropriate options, and pass this object to
! 8034: a RE constructor. Example:
! 8035:
! 8036: RE_Options opt;
! 8037: opt.set_caseless(true);
! 8038: if (RE("HELLO", opt).PartialMatch("hello world")) ...
! 8039:
! 8040: RE_options has two constructors. The default constructor takes no argu-
! 8041: ments and creates a set of flags that are off by default. The optional
! 8042: parameter option_flags is to facilitate transfer of legacy code from C
! 8043: programs. This lets you do
! 8044:
! 8045: RE(pattern,
! 8046: RE_Options(PCRE_CASELESS|PCRE_MULTILINE)).PartialMatch(str);
! 8047:
! 8048: However, new code is better off doing
! 8049:
! 8050: RE(pattern,
! 8051: RE_Options().set_caseless(true).set_multiline(true))
! 8052: .PartialMatch(str);
! 8053:
! 8054: If you are going to pass one of the most used modifiers, there are some
! 8055: convenience functions that return a RE_Options class with the appropri-
! 8056: ate modifier already set: CASELESS(), UTF8(), MULTILINE(), DOTALL(),
! 8057: and EXTENDED().
! 8058:
! 8059: If you need to set several options at once, and you don't want to go
! 8060: through the pains of declaring a RE_Options object and setting several
! 8061: options, there is a parallel method that give you such ability on the
! 8062: fly. You can concatenate several set_xxxxx() member functions, since
! 8063: each of them returns a reference to its class object. For example, to
! 8064: pass PCRE_CASELESS, PCRE_EXTENDED, and PCRE_MULTILINE to a RE with one
! 8065: statement, you may write:
! 8066:
! 8067: RE(" ^ xyz \\s+ .* blah$",
! 8068: RE_Options()
! 8069: .set_caseless(true)
! 8070: .set_extended(true)
! 8071: .set_multiline(true)).PartialMatch(sometext);
! 8072:
! 8073:
! 8074: SCANNING TEXT INCREMENTALLY
! 8075:
! 8076: The "Consume" operation may be useful if you want to repeatedly match
! 8077: regular expressions at the front of a string and skip over them as they
! 8078: match. This requires use of the "StringPiece" type, which represents a
! 8079: sub-range of a real string. Like RE, StringPiece is defined in the
! 8080: pcrecpp namespace.
! 8081:
! 8082: Example: read lines of the form "var = value" from a string.
! 8083: string contents = ...; // Fill string somehow
! 8084: pcrecpp::StringPiece input(contents); // Wrap in a StringPiece
! 8085:
! 8086: string var;
! 8087: int value;
! 8088: pcrecpp::RE re("(\\w+) = (\\d+)\n");
! 8089: while (re.Consume(&input, &var, &value)) {
! 8090: ...;
! 8091: }
! 8092:
! 8093: Each successful call to "Consume" will set "var/value", and also
! 8094: advance "input" so it points past the matched text.
! 8095:
! 8096: The "FindAndConsume" operation is similar to "Consume" but does not
! 8097: anchor your match at the beginning of the string. For example, you
! 8098: could extract all words from a string by repeatedly calling
! 8099:
! 8100: pcrecpp::RE("(\\w+)").FindAndConsume(&input, &word)
! 8101:
! 8102:
! 8103: PARSING HEX/OCTAL/C-RADIX NUMBERS
! 8104:
! 8105: By default, if you pass a pointer to a numeric value, the corresponding
! 8106: text is interpreted as a base-10 number. You can instead wrap the
! 8107: pointer with a call to one of the operators Hex(), Octal(), or CRadix()
! 8108: to interpret the text in another base. The CRadix operator interprets
! 8109: C-style "0" (base-8) and "0x" (base-16) prefixes, but defaults to
! 8110: base-10.
! 8111:
! 8112: Example:
! 8113: int a, b, c, d;
! 8114: pcrecpp::RE re("(.*) (.*) (.*) (.*)");
! 8115: re.FullMatch("100 40 0100 0x40",
! 8116: pcrecpp::Octal(&a), pcrecpp::Hex(&b),
! 8117: pcrecpp::CRadix(&c), pcrecpp::CRadix(&d));
! 8118:
! 8119: will leave 64 in a, b, c, and d.
! 8120:
! 8121:
! 8122: REPLACING PARTS OF STRINGS
! 8123:
! 8124: You can replace the first match of "pattern" in "str" with "rewrite".
! 8125: Within "rewrite", backslash-escaped digits (\1 to \9) can be used to
! 8126: insert text matching corresponding parenthesized group from the pat-
! 8127: tern. \0 in "rewrite" refers to the entire matching text. For example:
! 8128:
! 8129: string s = "yabba dabba doo";
! 8130: pcrecpp::RE("b+").Replace("d", &s);
! 8131:
! 8132: will leave "s" containing "yada dabba doo". The result is true if the
! 8133: pattern matches and a replacement occurs, false otherwise.
! 8134:
! 8135: GlobalReplace is like Replace except that it replaces all occurrences
! 8136: of the pattern in the string with the rewrite. Replacements are not
! 8137: subject to re-matching. For example:
! 8138:
! 8139: string s = "yabba dabba doo";
! 8140: pcrecpp::RE("b+").GlobalReplace("d", &s);
! 8141:
! 8142: will leave "s" containing "yada dada doo". It returns the number of
! 8143: replacements made.
! 8144:
! 8145: Extract is like Replace, except that if the pattern matches, "rewrite"
! 8146: is copied into "out" (an additional argument) with substitutions. The
! 8147: non-matching portions of "text" are ignored. Returns true iff a match
! 8148: occurred and the extraction happened successfully; if no match occurs,
! 8149: the string is left unaffected.
! 8150:
! 8151:
! 8152: AUTHOR
! 8153:
! 8154: The C++ wrapper was contributed by Google Inc.
! 8155: Copyright (c) 2007 Google Inc.
! 8156:
! 8157:
! 8158: REVISION
! 8159:
! 8160: Last updated: 17 March 2009
! 8161: Minor typo fixed: 25 July 2011
! 8162: ------------------------------------------------------------------------------
! 8163:
! 8164:
! 8165: PCRESAMPLE(3) PCRESAMPLE(3)
! 8166:
! 8167:
! 8168: NAME
! 8169: PCRE - Perl-compatible regular expressions
! 8170:
! 8171:
! 8172: PCRE SAMPLE PROGRAM
! 8173:
! 8174: A simple, complete demonstration program, to get you started with using
! 8175: PCRE, is supplied in the file pcredemo.c in the PCRE distribution. A
! 8176: listing of this program is given in the pcredemo documentation. If you
! 8177: do not have a copy of the PCRE distribution, you can save this listing
! 8178: to re-create pcredemo.c.
! 8179:
! 8180: The program compiles the regular expression that is its first argument,
! 8181: and matches it against the subject string in its second argument. No
! 8182: PCRE options are set, and default character tables are used. If match-
! 8183: ing succeeds, the program outputs the portion of the subject that
! 8184: matched, together with the contents of any captured substrings.
! 8185:
! 8186: If the -g option is given on the command line, the program then goes on
! 8187: to check for further matches of the same regular expression in the same
! 8188: subject string. The logic is a little bit tricky because of the possi-
! 8189: bility of matching an empty string. Comments in the code explain what
! 8190: is going on.
! 8191:
! 8192: If PCRE is installed in the standard include and library directories
! 8193: for your operating system, you should be able to compile the demonstra-
! 8194: tion program using this command:
! 8195:
! 8196: gcc -o pcredemo pcredemo.c -lpcre
! 8197:
! 8198: If PCRE is installed elsewhere, you may need to add additional options
! 8199: to the command line. For example, on a Unix-like system that has PCRE
! 8200: installed in /usr/local, you can compile the demonstration program
! 8201: using a command like this:
! 8202:
! 8203: gcc -o pcredemo -I/usr/local/include pcredemo.c \
! 8204: -L/usr/local/lib -lpcre
! 8205:
! 8206: In a Windows environment, if you want to statically link the program
! 8207: against a non-dll pcre.a file, you must uncomment the line that defines
! 8208: PCRE_STATIC before including pcre.h, because otherwise the pcre_mal-
! 8209: loc() and pcre_free() exported functions will be declared
! 8210: __declspec(dllimport), with unwanted results.
! 8211:
! 8212: Once you have compiled and linked the demonstration program, you can
! 8213: run simple tests like this:
! 8214:
! 8215: ./pcredemo 'cat|dog' 'the cat sat on the mat'
! 8216: ./pcredemo -g 'cat|dog' 'the dog sat on the cat'
! 8217:
! 8218: Note that there is a much more comprehensive test program, called
! 8219: pcretest, which supports many more facilities for testing regular
! 8220: expressions and the PCRE library. The pcredemo program is provided as a
! 8221: simple coding example.
! 8222:
! 8223: If you try to run pcredemo when PCRE is not installed in the standard
! 8224: library directory, you may get an error like this on some operating
! 8225: systems (e.g. Solaris):
! 8226:
! 8227: ld.so.1: a.out: fatal: libpcre.so.0: open failed: No such file or
! 8228: directory
! 8229:
! 8230: This is caused by the way shared library support works on those sys-
! 8231: tems. You need to add
! 8232:
! 8233: -R/usr/local/lib
! 8234:
! 8235: (for example) to the compile command to get round this problem.
! 8236:
! 8237:
! 8238: AUTHOR
! 8239:
! 8240: Philip Hazel
! 8241: University Computing Service
! 8242: Cambridge CB2 3QH, England.
! 8243:
! 8244:
! 8245: REVISION
! 8246:
! 8247: Last updated: 17 November 2010
! 8248: Copyright (c) 1997-2010 University of Cambridge.
! 8249: ------------------------------------------------------------------------------
! 8250: PCRELIMITS(3) PCRELIMITS(3)
! 8251:
! 8252:
! 8253: NAME
! 8254: PCRE - Perl-compatible regular expressions
! 8255:
! 8256:
! 8257: SIZE AND OTHER LIMITATIONS
! 8258:
! 8259: There are some size limitations in PCRE but it is hoped that they will
! 8260: never in practice be relevant.
! 8261:
! 8262: The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
! 8263: is compiled with the default internal linkage size of 2. If you want to
! 8264: process regular expressions that are truly enormous, you can compile
! 8265: PCRE with an internal linkage size of 3 or 4 (see the README file in
! 8266: the source distribution and the pcrebuild documentation for details).
! 8267: In these cases the limit is substantially larger. However, the speed
! 8268: of execution is slower.
! 8269:
! 8270: All values in repeating quantifiers must be less than 65536.
! 8271:
! 8272: There is no limit to the number of parenthesized subpatterns, but there
! 8273: can be no more than 65535 capturing subpatterns.
! 8274:
! 8275: There is a limit to the number of forward references to subsequent sub-
! 8276: patterns of around 200,000. Repeated forward references with fixed
! 8277: upper limits, for example, (?2){0,100} when subpattern number 2 is to
! 8278: the right, are included in the count. There is no limit to the number
! 8279: of backward references.
! 8280:
! 8281: The maximum length of name for a named subpattern is 32 characters, and
! 8282: the maximum number of named subpatterns is 10000.
! 8283:
! 8284: The maximum length of a subject string is the largest positive number
! 8285: that an integer variable can hold. However, when using the traditional
! 8286: matching function, PCRE uses recursion to handle subpatterns and indef-
! 8287: inite repetition. This means that the available stack space may limit
! 8288: the size of a subject string that can be processed by certain patterns.
! 8289: For a discussion of stack issues, see the pcrestack documentation.
! 8290:
! 8291:
! 8292: AUTHOR
! 8293:
! 8294: Philip Hazel
! 8295: University Computing Service
! 8296: Cambridge CB2 3QH, England.
! 8297:
! 8298:
! 8299: REVISION
! 8300:
! 8301: Last updated: 30 November 2011
! 8302: Copyright (c) 1997-2011 University of Cambridge.
! 8303: ------------------------------------------------------------------------------
! 8304:
! 8305:
! 8306: PCRESTACK(3) PCRESTACK(3)
! 8307:
! 8308:
! 8309: NAME
! 8310: PCRE - Perl-compatible regular expressions
! 8311:
! 8312:
! 8313: PCRE DISCUSSION OF STACK USAGE
! 8314:
! 8315: When you call pcre_exec(), it makes use of an internal function called
! 8316: match(). This calls itself recursively at branch points in the pattern,
! 8317: in order to remember the state of the match so that it can back up and
! 8318: try a different alternative if the first one fails. As matching pro-
! 8319: ceeds deeper and deeper into the tree of possibilities, the recursion
! 8320: depth increases. The match() function is also called in other circum-
! 8321: stances, for example, whenever a parenthesized sub-pattern is entered,
! 8322: and in certain cases of repetition.
! 8323:
! 8324: Not all calls of match() increase the recursion depth; for an item such
! 8325: as a* it may be called several times at the same level, after matching
! 8326: different numbers of a's. Furthermore, in a number of cases where the
! 8327: result of the recursive call would immediately be passed back as the
! 8328: result of the current call (a "tail recursion"), the function is just
! 8329: restarted instead.
! 8330:
! 8331: The above comments apply when pcre_exec() is run in its normal inter-
! 8332: pretive manner. If the pattern was studied with the PCRE_STUDY_JIT_COM-
! 8333: PILE option, and just-in-time compiling was successful, and the options
! 8334: passed to pcre_exec() were not incompatible, the matching process uses
! 8335: the JIT-compiled code instead of the match() function. In this case,
! 8336: the memory requirements are handled entirely differently. See the pcre-
! 8337: jit documentation for details.
! 8338:
! 8339: The pcre_dfa_exec() function operates in an entirely different way, and
! 8340: uses recursion only when there is a regular expression recursion or
! 8341: subroutine call in the pattern. This includes the processing of asser-
! 8342: tion and "once-only" subpatterns, which are handled like subroutine
! 8343: calls. Normally, these are never very deep, and the limit on the com-
! 8344: plexity of pcre_dfa_exec() is controlled by the amount of workspace it
! 8345: is given. However, it is possible to write patterns with runaway infi-
! 8346: nite recursions; such patterns will cause pcre_dfa_exec() to run out of
! 8347: stack. At present, there is no protection against this.
! 8348:
! 8349: The comments that follow do NOT apply to pcre_dfa_exec(); they are rel-
! 8350: evant only for pcre_exec() without the JIT optimization.
! 8351:
! 8352: Reducing pcre_exec()'s stack usage
! 8353:
! 8354: Each time that match() is actually called recursively, it uses memory
! 8355: from the process stack. For certain kinds of pattern and data, very
! 8356: large amounts of stack may be needed, despite the recognition of "tail
! 8357: recursion". You can often reduce the amount of recursion, and there-
! 8358: fore the amount of stack used, by modifying the pattern that is being
! 8359: matched. Consider, for example, this pattern:
! 8360:
! 8361: ([^<]|<(?!inet))+
! 8362:
! 8363: It matches from wherever it starts until it encounters "<inet" or the
! 8364: end of the data, and is the kind of pattern that might be used when
! 8365: processing an XML file. Each iteration of the outer parentheses matches
! 8366: either one character that is not "<" or a "<" that is not followed by
! 8367: "inet". However, each time a parenthesis is processed, a recursion
! 8368: occurs, so this formulation uses a stack frame for each matched charac-
! 8369: ter. For a long string, a lot of stack is required. Consider now this
! 8370: rewritten pattern, which matches exactly the same strings:
! 8371:
! 8372: ([^<]++|<(?!inet))+
! 8373:
! 8374: This uses very much less stack, because runs of characters that do not
! 8375: contain "<" are "swallowed" in one item inside the parentheses. Recur-
! 8376: sion happens only when a "<" character that is not followed by "inet"
! 8377: is encountered (and we assume this is relatively rare). A possessive
! 8378: quantifier is used to stop any backtracking into the runs of non-"<"
! 8379: characters, but that is not related to stack usage.
! 8380:
! 8381: This example shows that one way of avoiding stack problems when match-
! 8382: ing long subject strings is to write repeated parenthesized subpatterns
! 8383: to match more than one character whenever possible.
! 8384:
! 8385: Compiling PCRE to use heap instead of stack for pcre_exec()
! 8386:
! 8387: In environments where stack memory is constrained, you might want to
! 8388: compile PCRE to use heap memory instead of stack for remembering back-
! 8389: up points when pcre_exec() is running. This makes it run a lot more
! 8390: slowly, however. Details of how to do this are given in the pcrebuild
! 8391: documentation. When built in this way, instead of using the stack, PCRE
! 8392: obtains and frees memory by calling the functions that are pointed to
! 8393: by the pcre_stack_malloc and pcre_stack_free variables. By default,
! 8394: these point to malloc() and free(), but you can replace the pointers to
! 8395: cause PCRE to use your own functions. Since the block sizes are always
! 8396: the same, and are always freed in reverse order, it may be possible to
! 8397: implement customized memory handlers that are more efficient than the
! 8398: standard functions.
! 8399:
! 8400: Limiting pcre_exec()'s stack usage
! 8401:
! 8402: You can set limits on the number of times that match() is called, both
! 8403: in total and recursively. If a limit is exceeded, pcre_exec() returns
! 8404: an error code. Setting suitable limits should prevent it from running
! 8405: out of stack. The default values of the limits are very large, and
! 8406: unlikely ever to operate. They can be changed when PCRE is built, and
! 8407: they can also be set when pcre_exec() is called. For details of these
! 8408: interfaces, see the pcrebuild documentation and the section on extra
! 8409: data for pcre_exec() in the pcreapi documentation.
! 8410:
! 8411: As a very rough rule of thumb, you should reckon on about 500 bytes per
! 8412: recursion. Thus, if you want to limit your stack usage to 8Mb, you
! 8413: should set the limit at 16000 recursions. A 64Mb stack, on the other
! 8414: hand, can support around 128000 recursions.
! 8415:
! 8416: In Unix-like environments, the pcretest test program has a command line
! 8417: option (-S) that can be used to increase the size of its stack. As long
! 8418: as the stack is large enough, another option (-M) can be used to find
! 8419: the smallest limits that allow a particular pattern to match a given
! 8420: subject string. This is done by calling pcre_exec() repeatedly with
! 8421: different limits.
! 8422:
! 8423: Changing stack size in Unix-like systems
! 8424:
! 8425: In Unix-like environments, there is not often a problem with the stack
! 8426: unless very long strings are involved, though the default limit on
! 8427: stack size varies from system to system. Values from 8Mb to 64Mb are
! 8428: common. You can find your default limit by running the command:
! 8429:
! 8430: ulimit -s
! 8431:
! 8432: Unfortunately, the effect of running out of stack is often SIGSEGV,
! 8433: though sometimes a more explicit error message is given. You can nor-
! 8434: mally increase the limit on stack size by code such as this:
! 8435:
! 8436: struct rlimit rlim;
! 8437: getrlimit(RLIMIT_STACK, &rlim);
! 8438: rlim.rlim_cur = 100*1024*1024;
! 8439: setrlimit(RLIMIT_STACK, &rlim);
! 8440:
! 8441: This reads the current limits (soft and hard) using getrlimit(), then
! 8442: attempts to increase the soft limit to 100Mb using setrlimit(). You
! 8443: must do this before calling pcre_exec().
! 8444:
! 8445: Changing stack size in Mac OS X
! 8446:
! 8447: Using setrlimit(), as described above, should also work on Mac OS X. It
! 8448: is also possible to set a stack size when linking a program. There is a
! 8449: discussion about stack sizes in Mac OS X at this web site:
! 8450: http://developer.apple.com/qa/qa2005/qa1419.html.
! 8451:
! 8452:
! 8453: AUTHOR
! 8454:
! 8455: Philip Hazel
! 8456: University Computing Service
! 8457: Cambridge CB2 3QH, England.
! 8458:
! 8459:
! 8460: REVISION
! 8461:
! 8462: Last updated: 26 August 2011
! 8463: Copyright (c) 1997-2011 University of Cambridge.
! 8464: ------------------------------------------------------------------------------
! 8465:
! 8466:
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